A power transmission device comprising: a pin gear having a plurality of power transmission pins that are relatively moved corresponding to a tooth shape formed on an outer gear; a motor arranged in a direction crossing a rotation axis of the pin gear, and generating power to rotate the pin gear; and a motion transfer unit connected to the pin gear and the motor between the pin gear and the motor, and transferring a rotational motion of the motor to a rotational motion of the pin gear, characterized in that: the motion transfer unit comprises: a unit housing; and a built-in deceleration module provided in the unit housing, and decelerating a rotation force of the motor and transferring a decelerated rotational force to the pin gear., wherein the built-in deceleration module comprises: a drive bevel gear inserted in a first opening formed in the unit housing in a direction crossing the rotation axis of the pin gear, and rotated by the motor; a driven bevel gear inserted in a second opening communicating with the first opening and formed in a direction crossing the first opening, wherein the drive bevel gear is connected to the driven bevel gear in an area of the second opening, and decelerating a rotation force of the drive bevel gear by being engaged with the drive bevel gear, a drive bevel gear allowance adjustment member that is coupled to the drive bevel gear by the first coupling portion by contacting the drive bevel gear fixing member and adjusts allowance in an axial direction of the drive bevel gear, and a driven bevel gear allowance adjustment member that is connected to the driven bevel gear and adjusts assembly position allowance of the driven bevel gear. a pin gear having a plurality of power transmission pins that are relatively moved corresponding to a tooth shape formed on an outer gear; a motor arranged in a direction crossing a rotation axis of the pin gear, and generating power to rotate the pin gear; and a motion transfer unit connected to the pin gear and the motor between the pin gear and the motor, and transferring a rotational motion of the motor to a rotational motion of the pin gear, characterized in that: the motion transfer unit comprises: a unit housing; and a built-in deceleration module provided in the unit housing, and decelerating a rotation force of the motor and transferring a decelerated rotational force to the pin gear., wherein the built-in deceleration module comprises: a drive bevel gear inserted in a first opening formed in the unit housing in a direction crossing the rotation axis of the pin gear, and rotated by the motor; a driven bevel gear inserted in a second opening communicating with the first opening and formed in a direction crossing the first opening, wherein the drive bevel gear is connected to the driven bevel gear in an area of the second opening, and decelerating a rotation force of the drive bevel gear by being engaged with the drive bevel gear, a drive bevel gear allowance adjustment member that is coupled to the drive bevel gear by the first coupling portion by contacting the drive bevel gear fixing member and adjusts allowance in an axial direction of the drive bevel gear, and a driven bevel gear allowance adjustment member that is connected to the driven bevel gear and adjusts assembly position allowance of the driven bevel gear. a unit housing; and a built-in deceleration module provided in the unit housing, and decelerating a rotation force of the motor and transferring a decelerated rotational force to the pin gear., wherein the built-in deceleration module comprises: a drive bevel gear inserted in a first opening formed in the unit housing in a direction crossing the rotation axis of the pin gear, and rotated by the motor; a driven bevel gear inserted in a second opening communicating with the first opening and formed in a direction crossing the first opening, wherein the drive bevel gear is connected to the driven bevel gear in an area of the second opening, and decelerating a rotation force of the drive bevel gear by being engaged with the drive bevel gear, a drive bevel gear allowance adjustment member that is coupled to the drive bevel gear by the first coupling portion by contacting the drive bevel gear fixing member and adjusts allowance in an axial direction of the drive bevel gear, and a driven bevel gear allowance adjustment member that is connected to the driven bevel gear and adjusts assembly position allowance of the driven bevel gear. a drive bevel gear inserted in a first opening formed in the unit housing in a direction crossing the rotation axis of the pin gear, and rotated by the motor; a driven bevel gear inserted in a second opening communicating with the first opening and formed in a direction crossing the first opening, wherein the drive bevel gear is connected to the driven bevel gear in an area of the second opening, and decelerating a rotation force of the drive bevel gear by being engaged with the drive bevel gear, a drive bevel gear allowance adjustment member that is coupled to the drive bevel gear by the first coupling portion by contacting the drive bevel gear fixing member and adjusts allowance in an axial direction of the drive bevel gear, and a driven bevel gear allowance adjustment member that is connected to the driven bevel gear and adjusts assembly position allowance of the driven bevel gear.