AN auxiliary police sergeant was yesterday sentenced to three months' jail after being convicted of assaulting a man with his revolver during an identity check in the street. Kwun Tong magistrate Mr John Saunders had heard that Chan Hon-wai, 29, had stopped Mr Tam Chi-kuen on December 20, 1991, at about 8.30 pm, near Hing Tin estate car park. Mr Tam sustained a head wound which required three stitches, but Chan claimed this was the result of an accident, not an assault. The case was prosecuted after investigation by the Complaints Against Police Office (CAPO). Passing sentence yesterday, Mr Saunders said: ''An assault by a police officer in the execution of his duty on a citizen is a serious matter which constitutes an abuse of power and brings the reputation of the force into disrepute.'' While police have the right to stop and check people's identity cards, it must be exercised with care and consideration if it is to be respected by the community as an effective tool in crime prevention, the magistrate said. When exception is taken to a check, it is the police officer's duty to handle the situation diplomatically and not lose his temper, he added. He accepted mitigation from Mr Simon Westbrook, for Chan, that the injury was caused by one blow and other blows were not significant enough to leave a mark. He also accepted that the assault arose from Chan's anger at the manner in which Mr Tam responded to his lawful request to inspect his identity card. He said Chan was successful at his job and had a good record with the police, having been commended and promoted to sergeant. ''It is a tragedy that this otherwise good man should have marred his record with a momentary loss of temper,'' he said. In a four-day hearing earlier this month, Mr Tam testified that Chan remonstrated with him for causing him inconvenience and they swore at each other. Chan became angry, spun him round, pushed him against a hoarding and hit him on the head with his revolver. In his defence, Chan claimed Mr Tam became angry when he explained he had stopped him because he matched a description of someone who had indecently exposed himself earlier. He then punched Chan in the chest. As he tried to arrest Mr Tam, he found Mr Tam trying to remove his pistol. He tried to stop him and the two struggled, with Chan eventually succeeding in wresting the gun from Mr Tam, but as he did so, the gun flew up in the air, and down accidentally hitting Mr Tam on the head. The magistrate said he found Mr Tam a credible witness, whereas Chan's evidence was not consistent with his earlier statements. He said he found Chan's description of how the injury occurred to be ''illogical and unbelievable''. Medical evidence led by Senior Crown Counsel, Mr Peter Lavac, on the injury to Chan's chest was not consistent with the punch described by Chan in evidence, but was more likely the result of a glancing blow. It is not the first time a police officer has been prosecuted after investigations by CAPO. Criminal proceedings were taken against four police officers last year, two of whom were imprisoned. A third case was dismissed, while in the fourth, the crown offered no evidence. In 1991, three officers were imprisoned after investigations by CAPO, while a fourth case was dismissed. In 1992, two officers were sentenced to prison while four others were acquitted.