An engine (5) has a gas supply unit (10) which includes a piston (12) and supplies a drive combustion gas (71) in accordance with a reciprocating linear motion of the piston. The gas supply unit further includes: a cylinder (11) in which the piston reciprocates; a first combustion chamber (31) and a second combustion chamber (32) which are disposed on both sides of the piston in the cylinder; a common gas output port (17) for supplying a high-pressure combustion gas generated in the first combustion chamber for driving purpose; a first rod (13a) connected to the piston through the first combustion chamber; and a first piston control unit (60) for controlling the position of the piston outside the cylinder via the first rod.