The pilot reported that he joined downwind for runway 34, and completed downwind checks. He stated that normal landing gear extension occurred when he selected the undercarriage down, and a normal gear down indication was obtained (three green lights). The pilot stated that on late final approach the gear warning horn sounded when he reduced the throttle settings and he then noticed there were no green lights. He said that he immediately went around, and established the aircraft in the circuit at 1000 feet. The pilot stated that he extended and retracted the landing gear several times, and on each occasion the gear operated normally and normal indications were obtained. A second approach was then commenced, and the pilot said that the lights were monitored all the way to touchdown. He reported that the landing was normal, but that shortly after touchdown the right wing began to sink. The pilot stated that he believed the right hand gear had failed, and that he attempted to recover the situation by applying full power and using aileron to level the wings with a view to going around, and proceeding to an aerodrome better equipped to handle an emergency landing. At this point the left hand gear and the nosegear collapsed, and the aircraft slid to a stop on the gravel runway. Marks on the runway showed that the aircraft had landed with the landing gear extended and that the gear retracted soon after touchdown. Subsequent examination of the aircraft found no evidence of any pre-existing defect which may have contributed to the development of the accident. The landing gear switch was found in the down position, and the gear actuating mechanism was found to be in an intermediate position. Damage to the mechanism appeared to be consistent with the gear being driven down with the weight of the aircraft on it. This could occur if the gear was inadvertently selected up during the landing and then down again. The pilot is adamant, however, that this did not occur. The cause of the landing gear retracting during the landing roll could not be determined.