A power transmission device is capable of correctly adjusting relative positions of two shafts in a direction of rotation even when the shafts are rotationally driven, and comprises a hollow, outer shaft, to one end of which a drive gear is fixed, an inner shaft, to one end of which a carrier is fixed, and a planetary gear mechanism. The planetary gear mechanism comprises a sun gear, planetary gears supported on a carrier, and a ring gear having internal teeth and external teeth, the internal teeth engaging with the planetary gears. A first gear and a second gear on an input shaft, respectively, engage with outer teeth of the drive gear and the ring gear. When the sun gear is made stationary and the input shaft is rotationally driven, the outer shaft and the inner shaft are rotated with the same number of revolutions in the same direction. When the sun gear is rotated, a position of the inner shaft in the direction of rotation is adjusted relative to the outer shaft.