PURPOSE: To easily manufacture and work a member and to reduce the cost by alternately conducting the pressurized air to both ends of a hammer piston for longitudinally reciprocating the same, after an arbor is constrained against the movement relative to the casing.

CONSTITUTION: Two forward and backward stop shoulders 40, 44 mounted on the outer periphery of an arbor and two stopper surfaces 38, 42 formed on the casings 34, 36 are contacted with one another to constrain the arbor against the longitudinal movement relative to the casings 34, 36. Then a drill bit 50 is mounted on a front edge of the casing 34, a hammer piston 70 is mounted on an inner peripheral surface of the arbor 12, and a rear sub 46 is mounted on a rear end of the casing 36. The pressurized air is alternately conducted to both ends of the hammer piston 70 by the arbor 12 for longitudinally reciprocating the hammer piston 70, and giving the impact to the rear end of the drill bit.