A landing approach was made for runway 24 with the gear selected down and the cockpit gear lights indicating green. As the aircraft slowed on the landing roll the left main gear green light went out and the gear unsafe light came on. The aircraft was gently slowed and a left turn started when the left main gear leg collapsed. Inspection of the main gear leg showed that the pin securing the top of the leg to the wing structure, had seized and then sheared. The pin then moved forward and allowed the top of the leg to detach from the wing support structure, and collapse. Significant Factor The following factor was considered relevant to the development of the accident: 1. The pin securing the top of the left main landing gear leg to the supporting structure seized and fractured. (The reason for the seizure was not determined.)