To stably protrude the edge of a bit device outward to maintain and exactly perform excavation by providing a recess part communicating with the bottom face of two bit devices in the case where the device is rotated and one end of both the bit devices is protruded outward only in a prescribed excavation amount from the outer peripheral face of the device.

Bit devices 52a, 52b are rotated by rotation of a hammer cylinder 30, and force of excavation advancement is received to protrude one end thereof in a prescribed amount from the outer periphery of a pipe 46 to excavate soil. At the time, the partial contact face of the linear end faces of both the bit devices 52a, 52b becomes a stopper and rotation is stopped. In the state, the bit devices 52a, 52b receive rotation force of the hammer cylinder 30 to perform excavation, half-circular recess parts provided on the bottom face communicate with both the bit devices 52a, 52b, and the center thereof is a circular recess part 58 positioned on the rotation shaft of a device 36. In hard ground, rock enters a recess part 58, in soft and weak ground, clay enters the recess part 58, dislocation of the linear end faces of the bit devices 52a, 52b is prevented, and rotation can be not performed at a half-way rotation position.