PURPOSE: To increase operability by relating a throttle provided on the throttle rod of a throttle mechanism to the rotating reverse bush of a reverse mechanism.

CONSTITUTION: A throttle 20 provided on the throttle rod 21 of a throttle mechanism 7 is provided in relation to the rotating reverse bush 24 of a reverse mechanism 8. In order to make an air motor 16 rotate to the right viewing from the left part, a throttle 20 is turned to the right direction and a rotating reverse bush 24 is set with its turning desired posture. Then, the air fed from an annular groove 37 is fed to the right rotation side feed pass 18 through a passage 47 to make an air motor 16 rotate to the right. At this time, the left rotation side feed pass 17 becomes an exhaust passage together with an exhaust passage 19, the air is fed to an exhaust passage 10 via a passage 46, an annular groove 38 and a passage 48 and exhausted through a silencer 12.