The present invention detects a position of an object in space, and stores therein the detected position by associating with image data. At least one side of the piece of solid wood is scanned by the at least one scanning unit as the at least one scanning unit moves together with the pusher. The grating exhibits a first radiative mode corresponding to a first intensity of the input optical signal, and a second radiative mode corresponding to a second intensity of the input optical signal. The bubbles of air are discharged outside the reservoir, thereby purging the hydraulic booster of air completely.