If the output signal a of the sensor 5 scanned by the clock pulse CP is, for example, a signal such as shown in FIG. 5(A), the signal a is applied to the gate circuit GATE. There, sampling is done by gate signals such as shown in FIGS. 5(B), (C), which are formed on the basis of the clock pulse CP, and output signals b, c corresponding to the photoelectric conversion elements P1 -P4 (??), P5 -P8 (