A power transmission device comprising: a plurality of power transfer pins mutually corresponding to a tooth shape formed on an outer gear to move the outer gear and having an arrangement structure of a circular shape; and a pin rotation support portion connected to the plurality of power transfer pins and rotatably supporting the plurality of power transfer pins; charactreized by: further comprising: an external rotor motor portion arranged inside in a radial direction of the pin rotation support portion and connected to the pin rotation support portion, and generating rotational power to rotate the pin rotation support portion arranged outside, wherein the external rotor motor portion comprises: a rotor connected to the pin rotation support portion inside in the radial direction of the pin rotation support portion and rotating with the pin rotation support portion; a stator fixedly arranged inside a radial direction of the rotor and rotating the rotor by an applied current; a fixed shaft arranged inside the stator; and an absolute position sensor coupled to an end portion of the fixed shaft and detecting absolute positions of the plurality of power transfer pins. a plurality of power transfer pins mutually corresponding to a tooth shape formed on an outer gear to move the outer gear and having an arrangement structure of a circular shape; and a pin rotation support portion connected to the plurality of power transfer pins and rotatably supporting the plurality of power transfer pins; charactreized by: further comprising: an external rotor motor portion arranged inside in a radial direction of the pin rotation support portion and connected to the pin rotation support portion, and generating rotational power to rotate the pin rotation support portion arranged outside, wherein the external rotor motor portion comprises: a rotor connected to the pin rotation support portion inside in the radial direction of the pin rotation support portion and rotating with the pin rotation support portion; a stator fixedly arranged inside a radial direction of the rotor and rotating the rotor by an applied current; a fixed shaft arranged inside the stator; and an absolute position sensor coupled to an end portion of the fixed shaft and detecting absolute positions of the plurality of power transfer pins. an external rotor motor portion arranged inside in a radial direction of the pin rotation support portion and connected to the pin rotation support portion, and generating rotational power to rotate the pin rotation support portion arranged outside, wherein the external rotor motor portion comprises: a rotor connected to the pin rotation support portion inside in the radial direction of the pin rotation support portion and rotating with the pin rotation support portion; a stator fixedly arranged inside a radial direction of the rotor and rotating the rotor by an applied current; a fixed shaft arranged inside the stator; and an absolute position sensor coupled to an end portion of the fixed shaft and detecting absolute positions of the plurality of power transfer pins. a rotor connected to the pin rotation support portion inside in the radial direction of the pin rotation support portion and rotating with the pin rotation support portion; a stator fixedly arranged inside a radial direction of the rotor and rotating the rotor by an applied current; a fixed shaft arranged inside the stator; and an absolute position sensor coupled to an end portion of the fixed shaft and detecting absolute positions of the plurality of power transfer pins.