While a load torque applied to a tool shaft is lower than a predetermined value, a moving member is maintained at a first position, and a sun gear is rotated integrally with an internal gear. When the load torque applied to the tool shaft reaches or exceeds the predetermined value, the moving member is moved to a second position, to thereby prohibit relative rotation of the internal gear and a gear case. A latch member is engaged in a catching portion of the moving member when the moving member is moved to the second position, to thereby prevent the moving member from moving back to the first position. In this manner, repetitive switching between speed reduction ratios can be prevented even when the load torque applied to the tool shaft fluctuates.