To reduce weight of an impact driver and to smoothly change hammer inertia even when a motor is rotating during working, while in a conventional impact driver, adjustment of the output has been made by changing the hammer inertia when a small screw is fastened because its energy generation is too large.

The impact driver is composed of a spindle which is turned by a motor and has a slanting cam groove carved on the outer peripheral surface thereof, a hammer which is rotatingly driven by the spindle and is movable in the axial direction, an anvil to which rotation and impact are given by the hammer, a spring which always pushes the hammer toward the anvil, a speed reducing unit arranged between the motor and the spindle, and a casing for containing the elements described above. A weight cam is connected to the hammer so as to turn in the radial direction. A ring which carries out locking and releasing of the weight cam by its axial movement is engaged with the outer periphery of the hammer so as to move in the axial direction.