To provide a power tool having a built-in continuously variable transmission which continuously performs the variable transmission by solving problems that only the transmission of about two stages of the high speed and the low speed is variable by the engagement of gears in a reduction gear for reducing the rotation of an electric motor in the power tool such as a power driver drill.

The built-in continuously variable transmission 10 comprises a drive V-pulley 11 on the electric motor 4 side and a driven V-pulley 12 on a spindle 7 side, and a V-belt 13 stretched between the V-pulleys 11 and 12, the V-pulleys 11 and 12 comprise a pair of pulley plates 11a, 11b, 12a and 12b which are displaced to each other in the axial direction with the V-belt 13 held therebetween, and either or each of the pulley plates facing each other in the V-pulleys 11 and 12 are advanced/retracted to/from each other by the rotational resistance added via the V-belt 13 to change the diameter of the pulleys.