A driving piston 7 is provided so as to be slidable in an up-down direction within a driving cylinder 6 disposed within a body 1. A movable sleeve is fitted on the outer peripheral side of the driving cylinder 6 on the cylinder so as to be movable in the up-down direction. When the movable sleeve 11 is moved upward to abut against a cylinder head 10 provided in the upper direction, a sealed combustion chamber 5 is formed. When combustible mixed gas is explosively burnt within the combustion chamber 5, high-pressure combustion gas acts on the driving piston 7 to thereby impulsively drive the piston, whereby a fastener is driven out by a driver 8 coupled on the lower surface side of the driving piston 7. Elastic members 15 are disposed between the movable sleeve 11 and the cylinder head 10.The elastic members 15 bias the movable sleeve 11 downward.