THE PILOT VERIFIED THAT THE LANDING GEAR WAS DOWN AND LOCKED SEVERAL TIMES DURING THE APPROACH. AFTER A NORMAL LANDING, THE LANDING GEAR SUDDENLY COLLAPSED DURING ROLLOUT. AN EXAMINATION OF AIRPLANE REVEALED THAT THE ATTACHMENT RIVETS OF THE LANDING GEAR TRANSMISSION HAD FAILED DUE TO UNDETERMINED REASONS, AND THE TRANSMISSION HAD COMPLETELY SEPARATED FROM THE ATTACH BRACKET. SOME OF THE RIVETS WERE SHEARED OUT PREVIOUS TO THE ACCIDENT FLIGHT.