A screw driver (100) which comprises a motor (110) and a rotation transmission mechanism (120) is provided. The rotation transmission mechanism (120) comprises a driving gear (125), a spindle (150), a roller (141) and a retainer (130). The retainer (130) moves the roller (140) in a circumference direction of the spindle (150) and thereby a position of the roller (140) between a transmittable position and a non-transmittable position is switched.