It is customary in Spain for high court judges to make a little money on the side giving private classes to law students. For the most part, they take cash and don't declare it to the taxman. To do otherwise would be bad form and in bad taste. Bad form because you'd be letting the side down; bad taste because you'd be seen as a contrary prig, flaunting your superior morality. You'd be committing the cardinal Spanish sin of trying to stand out in a crowd.

In a country where black money is to the economy what oil is to the motor car, judges go along with the rules of the game, just like everybody else. But there is one judge in Spain - and possibly just the one - who refuses to allow black money to besmirch his righteous hand. Or so we are told in the authorised biography of Baltazar Garzon, immodestly titled El Hombre Que Veia Amanecer (The Man Who Saw the Sun Rise).

Celebrated within Spain as the intrepid scourge of ETA and the drug mafias, Garzon is better known outside Spain as the man who tried to bring Pinochet to justice (and Kissinger, and Berlusconi, and the whole of Galtieri's military junta). He turned up dramatically on the international scene again this week when he announced that he would be sending a rogatory commission to Indonesia to investigate the whereabouts of an Indonesian radical Muslim leader, Parlindungan Siregar. Siregar used to reside in Madrid and is said to have run an al-Qaeda camp where hundreds of mujahedin guerrillas were trained.

Garzon, who was one of the candidates for this year's Nobel Peace Prize, has not escaped the charge that he is a publicity seeking prima donna. His critics, and there are many in Spain, never tire of saying that he hungers for media attention; that he has what some insist on seeing as an almost pathological need to be "el protagonista" (the star of the show). His defenders outnumber the critics, though. A recent opinion poll commissioned by El Mundo found "the superjudge", as he is irreverently known, to be the most admired man in Spain.

At 46, Garzon is without question a show-off. One need look no further than the final 64 pages of El Hombre. They contain "133 previously unpublished photographs" of the judge, charting everything from his infancy to mugshots of his mum and dad; from a panoramic of the small town in AndalucIa where he was born to his first communion; from the seminary where he imagined he was preparing to enter the priesthood (until in his teens "he discovered girls") to his days as a bearded student lefty. And then numerous shots of his shaking hands with Felipe Gonzalez, Helmut Kohl, the King of Spain, Johan Cruyff, Kofi Annan, David Trimble.

On Spanish television, the recurring news image is of a man walking down the wide stone steps of an imposing building with a long coat worn over the shoulders like a cape, conveying a deliberate sense of purposeful flourish. And if this theatrical air can annoy his compatriots, they forgive, in the end, the man who boasts that father of all Spanish attributes, a mighty pair of "cojones". "Say what you like about the judge," they say, "but he's got cojones of steel."

Just in case, though, he has a round-the-clock bodyguard. No one in Spain is under more permanent threat of assassination from more quarters. No one has accumulated a more impressive collection of extremely dangerous enemies. After all, almost every time there is a news report announcing a major Colombian cocaine bust, or the arrest of ETA suspects, or - more recently - al-Qaeda suspects, Garzon pops up, Pimpernel-like, as the investigating judge in charge. One of the things an investigating judge does is issue search warrants and detention orders. The essence of the job is to assemble testimony and rule whether there is sufficient evidence to press charges and go to trial. The reason why Garzon seems to end up with all the juicy cases is something of a mystery for those outside the inner sanctum of the Audiencia Nacional, but has something to do with the fondness of the police for his uncompromising approach to law enforcement. The top cops on the top cases make their first call to the top judge.

Similarly, it is his reputation that helps explain why cases involving high-profile non-Spaniards are always brought to him. Pinochet, whose extradition he sought from Britain, was the evil old man who got the ball rolling. The stumbling block was Britain, which turned down a request from Garzon to interview Richard Nixon's secretary of state as a witness in the case he was building against the retired Chilean dictator on charges of genocide, terrorism and torture. He brought the same charges against 98 Argentinian military officers, 12 of them members of the junta responsible for the notorious "disappearances" of the late Seventies. The Kissinger connection flowed directly from the Pinochet case. Nothing came of this heroic gesture either. Not in the courts, at any rate. But it did achieve a pleasingly salutary outcome: it sent a message to all tyrants everywhere that the world was watching them and that they stepped outside the protection of their own national boundaries at their peril.

As for Silvio Berlusconi, the prime minister of Italy, Garzon proposed the extraordinary step a year ago of having the Spanish government ask Italy to drop his immunity and bring him to Spain to face trial on a multimillion euro tax fraud allegedly involving his Spanish television interests.

It must be said that Jose MarIa Aznar, the right-of-centre Spanish prime minister, was no more going to hand over his Italian counterpart to the mercies of the superjudge than he was to agree to independence for the Basque country. But if Aznar was irritated with Garzon when he did that, he would not have been irritated for long. The most remarkable thing about Garzon is that he confounds people time after time; he evades all attempts to attach partisan labels to him.

For example, four years ago he was going after a large media group that happened to have close links to the Socialist Party, prompting people to say he was a running dog of the "fascist" Aznar. The baying against the judge was all the louder because in 1994 he had served briefly and unhappily in the decaying and corrupt government of Felipe Gonzalez, from which he quit in disgust. But he silenced his critics on the left for ever, and drove the real fascists into a frenzy, by asking for the head of General Augusto Pinochet.

Even ETA must be confused. It was he who fought to uncover the Spanish government's "dirty war" against Basque nationalism; he who put a former Socialist interior minister behind bars. But then it is also he who, quite apart from waging relentless war on ETA for more than a decade, issued the ruling that gave a green light to the Spanish parliament two months ago to ban ETA's political wing, Batasuna.

What drives him? Where does all that crusading energy come from?

First, obviously, there is a large component of vanity. He himself acknowledged on quitting the Gonzalez government, having failed in his stated quest to clean out the Socialist stables, that he had succumbed to the sin of pride. "More than naive, I have been proud," he confessed to friends. "I thought I alone could do it all."

Second, there's the Catholic upbringing. The fervour of the seminary years evidently left its mark. "In another age," said a man who knows him, "he would have been converting Indians in Mexico, gospel in one hand, sword in the other."

But above all, the god that Garzon worships, and in whose cause he has made it plain that he is prepared to give his life, is the law. There is a terrible solemnity in his belief that the rule of law will set you free. And that mixture of solemnity and ego, and a craving for justice so blind that he does not hesitate to go after the likes of Kissinger and Berlusconi, gives a wide-open target to those who wish to expose him to ridicule. In the manner of Don Quixote, he sometimes treads a fine line between the noble and the ludicrous. There may be something half-cracked about his caped crusader's zeal. But the world would be a lesser place if he did not exist.