An automatic gearshift device for a power tool includes a speed reduction mechanism that decelerates and transmits a rotary power of a motor, a ring gear that slides and moves to switch a speed reduction ratio, a cam member that moves the ring gear, a rotation body rotated in cooperation with the cam member, a position detector that detects the position of the ring gear, and a controller that controls the driving of the gearshift motor based on the value generated by the position detector. An adjustment gear forming the rotation body includes a slot used as a positioning mark at a predetermined location in the rotating direction. When the slot of the adjustment gear is aligned with a predetermined device side positioning reference, the rotation body is positioned in an initial position in a relative positional relationship with the ring gear.