A MENTALLY retarded man yesterday admitted abducting a five-year-old boy when he was on probation for taking two other boys away from their guardians. Cheung Sik-tim, 37, pleaded guilty to a count of abducting a child. By committing the new offence, Cheung breached a probation order imposed on him in July 1993. He was at the time placed on probation for a maximum term of three years in respect of the abduction of Tsang Kin-kay, then nearly three, and Chan Chiu-kan, then 31/2. According to a psychiatric report prepared in 1992, Cheung suffered moderate mental retardation. The court heard Cheung had a mental age of between three and six, having suffered from meningitis when he was young. And his intellectual functions were impaired after being knocked down by a car at the age of six. Defence counsel Wong Hay-yiu described the case as ''unusual and sad'' and said Cheung's elderly parents were unable to keep an eye on him 24 hours a day. In the present case, counsel said, Cheung got lost and bumped into a small child whom he later took away and played with. Prosecutor Anna Lai said Li Wa-shing took his son Chun-lung, five, to his workplace at 36 Jardine's Bazaar, Causeway Bay, on March 26 last year. As Chun-lung knew the place well, his father allowed him to wander unattended nearby. Chun-lung then walked to a Watson's shop, where Cheung approached him. Cheung claimed he knew Chun-lung's father and took the boy on an MTR and bus ride to the New Territories. ''They ate in a McDonald's food shop. Then they wandered in a public park until night-time and slept on an unknown staircase,'' Ms Lai said. The following morning, they returned to Hong Kong Island and wandered until night-time when they slept on a bus in a bus terminus. On March 28, they again went to various places before they went to a bank in Central in the afternoon. As Cheung had spent all his money, he told the boy to call on his father to obtain $300. At 4.30 pm, Chun-lung rang up his father at his office. Mr Li immediately made a report to the police. A constable rushed to the ground floor of the bank and arrested Cheung. Deputy Judge Yung called for a probation report on Cheung and remanded him in a psychiatric centre until sentencing on August 19.