A planar light source includes a large variable width of an irradiation angle of illumination light, a display device having a large variable width of an angle of field that uses the planar light source, a portable terminal device that uses the display device, and a ray direction switching element that is incorporated in the planar light source. One electrode of the capacitor element is connected to one storage node of a flip-flop circuit, and the other electrode is connected to the other storage node. It sends the message to a plurality of devices, and receives a response from at least two of the plurality of devices. The optical pick-up unit then can be incrementally moved away from the extreme position while the tracking error signal is monitored to locate an area of the optical media with tracks. The constructed prefilters provide equalization in both the forward and backward directions.