To prevent such a situation that a signal from a pixel in a dark state is output at a level shifted from an originally set level to deteriorate an image quality, and to improve the image quality. A plurality of impact hammers located within the milling chamber are fastened to and longitudinally disposed along a rotor assembly that is connected to a rotary driving mechanism. Each plate has an outer surface which engages a vertebra and a metallic inner curved surface which is shaped to slide over one of the curved surfaces of the core. The memory gate electrode and the switch gate electrodes extend in the same direction. This device comprises means for conducting a washing liquor past the sensor means and means for determining a certain contact time between the washing liquor and the sensor means.