The free-piston engine 10 includes a combustion space F for combusting an air-fuel mixture, a piston 12 capable of reciprocation between a most-compressed position and a most-expanded position, suction ports 14 for introducing outside air into the combustion space F, and exhaust ports 16 for directing the exhaust gas to the outside. The piston 12 extracts power by moving from the most-compressed position to the most-expanded position by a combustion explosive force and returns from the most-expanded position to the most-compressed position by the actuation of a piston drive device. Furthermore, the piston 12 opens the exhaust port 16 to the combustion space F when the piston 12 has reached the most-expanded position, whereas the piston 12 closes the exhaust ports 16 to the combustion space F when the piston is present in a different position.