Abstract: A rotation position detector includes a motor having a drive shaft extending along a first axis, a drive-side pulley that is connected to the drive shaft, a holder that holds a treatment tool to be inserted into a patient during surgery, the holder rotating in association with the drive-side pulley, a driven-side pulley that rotates around the first axis, a diameter of the drive-side pulley being smaller than a diameter of the driven-side pulley, a transmission belt that transmits a rotational drive of the drive-side pulley to the driven-side pulley, a rotary encoder that is provided on the first axis and detects a rotation angle of the driven-side pulley, and a controller that calculates a rotation position of the holder based on the rotation angle and based a pulley ratio of the drive-side pulley to the driven-side pulley, and controls the motor based on the rotation position.