A control method for a power assist device provided with an operation handle, a force sensor that detects an operation force applied to the operation handle and an orientation of the operation force, a robot arm, and an actuator. When the orientation of the operation force is detected to be within a predetermined angle range with respect to a preset advancing direction of the operation handle, the actuator is controlled so as to move the operation handle along the advancing direction by employing only a component of the operation force along the advancing direction; and when the orientation of the operation force is detected to be outside the predetermined angle range, the actuator is controlled to move the operation handle by the operation force applied to the operation handle and the orientation of the operation force.