PURPOSE: To avoid a large quantity of unburned gas exhausted into the atmosphere by providing a valve device for opening and closing a fuel inlet port and an outlet port in association with a trigger mechanism.

CONSTITUTION: When an actuator 21 in a trigger mechanism 22 is operated, a lug 23 lowers a valve stem 17 via a shoulder 24 and a shuttle 20. Therefore, a fuel inlet port 6 and an outlet port 12 formed in an end wall 18 of a valve device 10 are opened. Then, when the lug 23 deviates the shoulder 24, the shuttle 20 returns due to a spring 19, and fuel having flowed into a combustion chamber 2 from the fuel inlet port 6 is ignited by an ignition device 13. As a consequence, combusted gas is exhausted from the outlet port 12 while a piston 3 is lowered. Here, while gas is exhausted from an exhaust port 11, the piston 3 lowers a cylinder 4 to attack a resilient bumper 55 at a lowest position and return to an ignition position. Due to the operation of the valve device 10, fuel is completely combusted so that the exhaust of unburned gas into the atmosphere is suppressed.