A hammer drill has a mode change-over mechanism including a change link with a horizontally elongated slot, an eccentric pin projecting from a mode selector switch and penetrating the slot. The change link is moved vertically by rotating the mode selector switch, which in turn vertically moves a pair of change keys secured to a holder of the change link, thus either engaging or disengaging the change keys with or from a sleeve provided in a rotation transmission mechanism for transmitting the rotation of a motor to the tool holder. In the engagement position, the rotation of the motor of the hammer drill is transmitted to a tool holder while in the disengagement position, the rotation of the motor is not transmitted. Also, the eccentric pin is engaged with a square recess formed in a lock ring which is fitted around the tool holder and urged backward by a compression spring. The eccentric pin is horizontally moved by rotating the mode selector switch, which in turn moves the lock ring in the axial direction, thereby couple or de-couple the lock ring to or from the tool holder.