The pilot stated that he carried out a normal circuit at his destination. During the landing roll he selected the flaps up, and then inadvertently selected the landing gear up before realising his mistake and selecting the gear down again. The gear up selection occurred just as the aircraft was travelling over a hump in the strip and it is believed that this, combined with the pilot holding up elevator, caused the weight of the aircraft to come off the wheels. This resulted in the landing gear safety switch becoming ineffective and the gear commencing the retraction sequence. When the aircraft came to rest the nosegear was fully retracted, the left maingear was partially retracted and the right maingear was still down and locked. Subsequent inspection and testing of the landing gear system did not reveal any faults that could have contributed to the occurrence. The pilot stated that he believed he had mistakenly applied the after-takeoff checks instead of the after-landing checks and had selected the gear up instead of opening the cowl flaps. This accident was not the subject of an on-site investigation.