On January 24, 2003, about 1145 mountain standard time, a Mooney M20D, N6639U, experienced the collapse of its main landing gear during landing rollout on runway 05 at the Casa Grande Municipal Airport, Casa Grande, Arizona. The airplane was substantially damaged. Neither the private pilot, who operated the airplane, nor the passenger was injured. The airplane was being operated on a personal flight under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed. No flight plan had been filed for the local area flight that originated from Casa Grande about 1045. The pilot reported that during his approach for landing, he had checked three times to confirm that the landing gear was extended and the green landing gear down lights were illuminated. Everything appeared satisfactory, and he made a normal landing. As the airplane decelerated on the runway, the main landing gear collapsed. Thereafter, as the airplane veered off the runway it collided with a runway light. On February 7, 2003, the Federal Aviation Administration (FAA) coordinator reported that the airplane examination revealed substantial damage had resulted during the gear collapse/impact event. The pilot reported that he owns the accident airplane. The airplane had been operated for 4.77 hours since receiving its last annual inspection. The pilot holds an FAA airframe and powerplant mechanic certificate, and he indicated that the airplane had previously undergone maintenance to its landing gear. According to the pilot, during the mishap the landing gear collapsed due to an adjustment problem, which prevented the over-center linkage from completely locking. The FAA indicated that the pilot/mechanic had performed the landing gear related maintenance.