Electric power tool

An electric power tool includes a motor (1); a speed reducer (2) for transferring a rotational power of the motor (1) at a reduced speed; and a reduction ratio changing unit (6,60,61,62) for changing a reduction ratio of the speed reducer. The speed reduction mechanism (2) includes an axially slidable changeover member (7) and a gear member (5), the changeover member (7) being engaged with or disengaged from the gear member (5) depending on an axial slide position thereof. The reduction ratio changing unit (6,60,61,62) includes a shift actuator (6) for axially sliding the changeover member (7), a driving state detector (60) for detecting a driving state of the motor, a slide position detector (61) for detecting a slide position of the changeover member (7) and a controller (62) for driving the shift actuator (6) and for temporarily decreasing or increasing a rotational power of the motor (1) depending on detection results of the driving state detector (6) and the slide position detector (61), respectively.