And he should know. Semba, 61, retired from the Ehime Prefectural Police in March, after 36 years on the force. At 24 he had been the youngest officer in the history of the prefectural force to be promoted to the rank of sergeant, but he says his refusal to falsify expenses forms that were funnelled into a vast slush fund meant that he was never promoted again, was regularly transferred between unappealing assignments and had his handgun taken away on the grounds that he might kill himself or pose a danger to others.

In 2005 he held a news conference to air his allegations, and as a result, he was transferred by his bosses.

In 2007, Semba sued and received vindication when he won one million yen (HK$83,617) in damages from the Ehime government. 'The work transfer represents a retaliatory action against the plaintiff and was absolutely unreasonable,' the judge was quoted as saying in the Japan Times.

Even before his retirement, still as a sergeant, Semba was giving lectures across Japan detailing the goings-on within the police and he has written a number of books on the subject.

He was under constant pressure to leave the force, he says, but chose to resist until he reached retirement age. But he similarly refuses to let the matter quietly drop.

Semba alleges that the police systematically rack up 40 billion yen every year from falsified travel expenses and fictitious payments to individuals supposedly assisting the police in their investigations. Almost every officer in the country is involved in the scam, he claims, and they do not speak out because they are all busy climbing the ranks to get a larger share of the pie.

'The money is spent by senior officers on purchasing cars, buying homes and entertainment,' he said.

A spokesman for the National Police Agency in Tokyo declined to comment on the allegations.

The media helps the police protect its own when they break the law in more flagrant ways, he says. Semba is presently supporting the fight by bus driver Haruhiko Kataoka, 65, who in February completed a prison term for a crime that he is adamant he did not commit.

Kataoka's head-on collision with the forces of law and order began May 3, 2006, as he was driving a school bus in Kochi City. After pulling out of a parking lot - and observing all the appropriate safety precautions, he insists - a motorcycle driven by a member of the Kochi Prefectural Police drove into the side of his vehicle, killing the officer.

Kataoka is adamant that his vehicle was at a complete halt when it was struck - a claim backed up by teachers on the bus - but another police officer who happened to be passing arrested him on the spot. After being questioned for two days - and repeatedly told that the officer's death was his fault - Kataoka was released.

'It was only eight months later that I was given an opportunity to explain what had happened, after I was summoned to the Kochi District Prosecutors' office,' he said. 'But the description of the accident they gave me then was beyond belief.'

The prosecutors told Kataoka the accident had been entirely his fault and that he was being charged with professional negligence resulting in death. To support their case, the police showed him photos of tyre skid marks on the road.

'Since the bus was stopped, I told them, there was no way it could have made the skid marks,' he said. 'It was then that I realised I was in a very problematic situation.'

Still, he reasoned, he would at least be able to explain what had really happened in court. He says he 'had trust in Japan's trial system'. Instead, the testimony of the teachers on the bus were dismissed by the Kochi Local Court on the grounds that their comments 'lacked a realistic basis'.

The testimony provided by the police officer who had been passing, however, was perfectly acceptable because 'testimony by a fellow officer is not necessarily unreliable'.

The court also accepted the tyre skid marks put forward by the prosecution, which indicated the bus was moving at a speed of 14 km/h, while the motorcycle was travelling at between 30 km/h and 40 km/h. That contradicted another eye-witness statement that the police motorcycle was doing 60 km/h. The judge dismissed that suggestion as simply difficult to believe.

Kataoka was found guilty and sentenced to one year and four months in prison. An appeal based on discrepancies in the evidence was summarily rejected, with Kataoka saying that the judge, 'merely dismissed all the evidence that was unfavourable to the police and tried to cover up the criminal actions of the police against me'. The Supreme Court reacted in the same way.

'I believe the courts have discarded the very principles of the judicial system and are only trying to cover up the wrongful actions of the police,' Kataoka said.

'This case is not special at all and there have been many victims of criminal actions by the police and the failure of the powers that be to carry out full investigations. How can I put my faith in the justice system when the facts of a case are fabricated?'

To date, Semba has given 88 lectures on his experiences, the vast majority of them while he was still a serving officer. He was never formally disciplined for blowing the whistle, he believes, because the police do not want a court case in which all their dirty laundry can be aired in public.

Semba is clearly a thorn in the side of the force and he half-joked that it is 'a miracle that I am still alive'.

'If I was in a senior position in the police I would definitely eliminate Semba,' he said. 'I'm the police's worst enemy. But it is those who have already given up their lives that are the strongest.'