PURPOSE: To allow a system to recognize fluctuation in a grasping position owing to displacement of a grasped part.

CONSTITUTION: When a part 2 is grasped, a rotary disc 11a in the outer peripheral surface of which a saw teeth-like uneven surface part is formed is pressed against the surface of the part 2 and brought into contact therewith. When the grasping position of the part 2 is displayed in the rotation direction of a rotary disc 11a owing to displacement in a position, the rotary disc 11a is rotated in given one way. A microphoto sensor 15a detects the uneven surface part of the outer peripheral surface of the rotary disc along with rotation of the rotary disc 11a and a pulse signal responding to the detection is outputted. By counting pulse waveforms contained in a pulse signal, an amount of the fluctuation in a position, in the rotation direction of the rotary disc 11a, of the part 2 is determined through a given calculating method.