A working depth adjusting device includes an adjusting member, an intermediate member and an operation member. The adjusting member is supported on the front portion of the tool body and movable along the axis of the spindle, so that the position of the adjusting member can be adjusted relative to the tool bit with respect to a working depth direction. The intermediate member is rotatably supported on the adjusting member. The adjusting member moves in the working depth direction as the inmate member rotates. The operation member is operable to rotate relative to the intermediate member between a lock position and an unlock position. The intermediate member has a first engaging member. When the operation member is rotated to the lock position, the first engaging member engages the tool body, and the engagement of the first engaging member is maintained by the operation member, so that the working depth adjusting device can be prevented from being removed from the tool body. When the operation member is rotated to the unlock position, the first engaging member is disengaged from the tool body, so that the working depth adjusting device can be removed from the tool body.