To solve the problem wherein a screw comes out or is left untightened, in a screw driving machine including an air motor for rotating a driver bit and a piston for axially moving, due to a force opposite to the direction of a propelling force acting on the bottom face of the piston in a structure of causing compressed air in a cylinder upper chamber to flow into a return air chamber at the time of downward movement of the piston to use it as return air for upward movement of the piston.

When the piston 22 reaches the bottom dead center, a damper 26 is lowered by its thrust force, to make a ventilation chamber 33 for supplying the compression air to the air motor 50 communicate with a cylinder lower chamber to cause the return air pressure to act on the lower surface of the piston 22.