A combustion-powered nail gun including an arrangement for promoting a piston returning motion to its upper dead center. A combustion chamber frame is movable toward and away from a head cover at which a spark plug is provided. When the combustion chamber frame is in close contact with the head cover, a combustion chamber is provided among the combustion chamber frame, the head cover, a part of a cylinder and a piston. After the piston reaches the lower dead center as a result of combustion, a thermal vacuum is provided in the combustion chamber for allowing the piston to move toward the upper dead center. The combustion chamber frame has a through-hole, and a valve is provided for normally closing the through-hole. A solenoid is provided to selectively open the valve for leaking an increased pressure in the combustion chamber through the through-hole during the return stroke of the piston.