After a 13-hour interrogation in which he was denied food, water or a lawyer, he confessed. Then he burst into tears: 'I was just so scared.'

That false confession cost Sugaya dearly. He spent more than 17 years in jail for Mami Matsuda's 1990 murder before being freed in June in a rare judicial reversal that has sent shock waves through the justice system. Now aged 62, as he waits for the state officially to declare him innocent, reformers are rallying around his case, which they think might finally force change on Japan's much-criticised police and courts.

'I believe his release could do for Japan what the Guildford Four and Birmingham Six cases did for the UK,' says Sugaya's lawyer Hiroshi Sato. Those wrongful convictions led to a major overhaul of police methods and the establishment of the Criminal Cases Review Commission.

'Japan needs something similar to bring it into the modern world,' Sato adds.

Detectives followed Sugaya for a year in search of evidence to support their belief that he murdered Mami, whose body was dumped by a riverside in Ashikaga.

Their interest in him began after they discovered in preliminary checks that he shared the same B blood-type as the killer.

They found a cripplingly shy man whom they believed fitted the profile of someone who would commit such an awful crime. Middle-aged and divorced because of his inability to perform sexually, he was an avid consumer of pornography. His job driving a kindergarten bus put him in proximity with potential victims. All that was missing was evidence.

In June 1991, the police rifled through Sugaya's trash and found what they were looking for - a discarded tissue carrying his semen. They sent it to the National Research Institute for Police Science, which was pioneering the new but largely untested world of DNA forensics in Japan. The institute declared a match with semen on Mami's underwear.

Like many suspects, Sugaya later retracted his confession, but had little chance of proving his innocence in courts that overwhelmingly tilted towards the police and prosecutors.

Admissions of guilt are still legally highly prized and the police are allowed up to 23 days to interrogate a suspect. Defence lawyers rarely successfully challenge confessions in a system with a conviction rate of more than 99 per cent. Lawyers are not allowed to be present during interrogations, either before or after indictment. Suspects often allege psychological and physical mistreatment.

The system has come under fire from the Japan Bar Association, Human Rights Watch, the International Bar Association and the UN Committee on Torture, which in 2007 delivered a withering report on Japan's conviction rate.

Critics acknowledge that the police are mostly thorough and that ultimately Japan incarcerates far fewer people than most developed countries - about a third of the British rate. But once at the mercy of the prosecutors, Sugaya was almost helpless. Even his original lawyer believed he was guilty and abandoned him, says Sato. He was sentenced to life behind bars in July 1993.

In prison, he was beaten by a fellow inmate angered by his protestations of innocence. It was not until the offer of help from Sato, a DNA expert who agreed to work pro bono, that light appeared at the end of the tunnel. 'I told him to secretly put a hair into an envelope and we would do another DNA test,' Sato recalls.

Private DNA tests showed that there was no match with the DNA found on Mami. In October 1997, Sato submitted the new evidence to the Supreme Court, requesting a retrial. The request was denied in 2000. In February 2008, another petition was rejected.

Then, unexpectedly, in December last year the Tokyo High Court ordered a fresh DNA test. Neither Sato nor Sugaya knows why, but they suspect public pressure played a part.

Two years ago, broadcaster NTV began a rare media investigation into Mami's case, citing its similarities with four unsolved child murders in the area. Many now speculate that the police have missed a serial killer, who may never be caught: Japan's statute of limitations has already expired on the murders, a fact that infuriates Sugaya.

'There should be no statute of limitations on cases like this. It's totally unacceptable,' he says.

Sugaya's release has triggered a string of rare apologies, among them one from the head of the prefectural police force, Shoichiro Ishikawa. Sugaya, though bitter, has accepted them all but waits for the one that counts: the original police team that forced his confession. 'I want them to face me like men and say sorry.'

Television reporters recently doorstopped the now-retired officers, who could be heard snarling through letterboxes. The police have already hinted, Sato says, at a private apology - away from the prying eyes of the media.

Elsewhere, however, there are worrying signs of business as usual. Judge Hiroshi Yamura, freeing Sugaya in the High Court, rejected defence demands to summon the police and scientists from the original trial. Some time this year, there will be an embarrassing admission of systemic failure, but no investigation into why it occurred - a whitewash, Sato believes.

Still, he thinks the aftershocks may bring something long demanded by the Japan Federation of Bar Associations - curbs on Japan's much criticised system of long pretrial detentions and the videotaping of police confessions. The case has already forced the prosecution this month to agree to hand over audio tapes of Sugaya's interrogation.

Sugaya can expect compensation of about 12,500 yen (HK$1,000) for every day he spent in prison - about 70 million yen.

Even with that, though, restarting life will be tough. His parents are long dead and his siblings have severed ties in a bid to put the scandal behind them.