The pilot was taking some friends for a short flight before the end of daylight. Just prior to touchdown he became aware that the landing gear was not extended, and as he believed it was too late to attempt a go-around, he landed the aircraft with the landing gear retracted. The aircraft slid to a halt on the runway and all the occupants evacuated uninjured. The pilot reported that he believed he had not been distracted prior to landing and that he completed the prelanding checks but had not noticed that the landing gear was not extended. When the aircraft came to rest it blocked both runways. Due to the imminent arrival of two scheduled services the pilot advised off duty airport groundstaff, who organised the removal of the aircraft from the runway.