An electric power tool includes a cylindrical reducer case accommodating the speed reduction mechanism. The speed reduction mechanism includes a planetary gear train and a movable member which is axially slidable to be engaged with or disengaged from the planetary gear train. The electric power tool includes the reducer case including a slide hole formed through a sidewall of the reducer case and axially extended and a rotary plate which is rotatable around a periphery of the reducer case about the axis, the rotary plate including an operation slot formed axially obliquely and overlapped with the slide hole; a supporting member radially outwardly protruded from the movable member and extended through the slide hole and the operation slot; and a biasing unit for applying a pressing force to the supporting member in a moving direction of the supporting member when the rotary plate is rotated to a position.