The pilot stated that after an uneventful flight, he entered the traffic pattern at the destination airport. He extended the landing gear as the airplane was crossing mid-field on downwind for runway 23 (6,100 feet by 100 feet, asphalt). There was a right quartering headwind for landing. The pilot noted that he was focused on maintaining airspeed and establishing the proper crosswind landing attitude. As a result, he failed to check the landing gear position lights in order to verify that the gear was down and locked. After landing, the left main landing gear collapsed, followed by the right main landing gear. The airplane skidded to the left and came to rest in the grassy area adjacent to the runway. The pilot noted that the cockpit landing gear lever provided resistance although it was not fully in the down position. He added that there were no failures or malfunctions associated with the airplane prior to the accident.