On May 30, 2002, at 1722 eastern daylight time, a Cessna 210, N7PJ, registered to and operated by Air Carriers Inc., landed gear up at Sarasota/Bradenton International Airport in Sarasota, Florida. The cargo flight was operated by the air transport pilot under the provisions of Title 14 CFR Part 135 with an Instrument flight plan filed. Visual meteorological conditions prevailed at the time of the accident. The airplane sustained minor damage, and the air transport pilot received minor injuries. The cargo flight departed Hernando County Airport in Brooksville, Florida, at 1630. According to the pilot, on downwind he put the landing gear handle into the down and locked position. He did not recall if he had a down and lock indication light. Upon touchdown, the landing gear retracted and the airplane collided with the ground. Examination of the airplane revealed that the landing gear was in the up and locked position, and that there were no mechanical discrepancies found. The Cessna 210, before landing checklist states in part that the landing gear should be extended and that the pilot should visually observe that the main gear is down and that the green indicator light is on. The registered owner and pilot were requested to complete a pilot/operator report but have not responded.