A positioning control system is provided with a state-feedback control system with a state observer as a full-closed control system for driving and controlling a motor so that a load shaft, which is an output shaft of a strain wave gearing, is positioned at a target position on the basis of a load shaft position actually detected. The state observer estimates a motor shaft position and a motor velocity based on a control input for the motor and the load shaft position. The state-feedback control system feeds back the state quantity of the object of control using the load shaft position as well as estimated motor shaft position and estimated motor velocity obtained by the state observer. It is possible to suppress resonant vibration caused by angular transmission error in the strain wave gearing and perform highly accurate positioning.