This 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, a suction port (14) for introducing outside air into the combustion space (F), and an exhaust port (16) for directing the exhaust gas to the outside. Power is extracted by movement of the piston (12) from the most-compressed position to the most-expanded position by fuel explosive force and, meanwhile, the piston (12) returns from the most-expanded position to the most-compressed position through action of a piston drive device. Further, when the piston (12) reaches the most-expanded position, said piston opens the exhaust port (16) to the combustion space (F), and when in a different position, closes the exhaust port (16) to the combustion space (F).