An automatic gearshift device for a power tool comprising: a speed reduction mechanism including a gear member, wherein the speed reduction mechanism decelerates and transmits a rotary power of a driving source; a switching member that slides and moves to an engagement position where the switching member engages with the gear member of the speed reduction mechanism to switch a speed reduction ratio of the speed reduction mechanism; a gearshift motor; a cam member driven and rotated by the gearshift motor to move the switching member to the engagement position; a rotation body rotated in cooperation with the cam member when driven by the gearshift motor, wherein the rotation body is positioned in an initial position in a predetermined relative positional relationship with the switching member; a position detector that detects movement of the switching member to the engagement position and generates a value indicating that the switching member has moved to the engagement position when the rotation body is rotated from the initial position by a predetermined rotation angle; and a controller that controls the driving of the gearshift motor based on the value generated by the position detector, characterized in that the rotation body includes a positioning mark set at a predetermined position in a rotating direction of the rotation body, and when the positioning mark is located at an initial position which is aligned with a predetermined device side positioning reference, the position detector is adapted to output an initial value indicating that the switching member is located in an initial position of the switching member. a speed reduction mechanism including a gear member, wherein the speed reduction mechanism decelerates and transmits a rotary power of a driving source; a switching member that slides and moves to an engagement position where the switching member engages with the gear member of the speed reduction mechanism to switch a speed reduction ratio of the speed reduction mechanism; a gearshift motor; a cam member driven and rotated by the gearshift motor to move the switching member to the engagement position; a rotation body rotated in cooperation with the cam member when driven by the gearshift motor, wherein the rotation body is positioned in an initial position in a predetermined relative positional relationship with the switching member; a position detector that detects movement of the switching member to the engagement position and generates a value indicating that the switching member has moved to the engagement position when the rotation body is rotated from the initial position by a predetermined rotation angle; and a controller that controls the driving of the gearshift motor based on the value generated by the position detector, characterized in that the rotation body includes a positioning mark set at a predetermined position in a rotating direction of the rotation body, and when the positioning mark is located at an initial position which is aligned with a predetermined device side positioning reference, the position detector is adapted to output an initial value indicating that the switching member is located in an initial position of the switching member.