PURPOSE: To eliminate the need for an offset for an output, and to determine white or black easily by rewriting the information of a pre-scanning line written in a buffer memory only when an output level from a differential amplifier is positive or negative.

CONSTITUTION: One-dimensional photodetecting element array 21 runs parallel with one-dimensional photodetecting element array 22 without shielding light, the array 21 is scanned and the array 22 is scanned, and their optical information are read. Reading outputs S2i and S1i regarding a number i picture element P2i and the same number picture element P1i of a pre-scanning line are inputted to a differential amplifier 18. The amplifier 18 takes, amplifies and outputs a difference between these two signals. When the picture element P1i is black (white) and P2i white (black), an output from the amplifier 18 reaches positive (negative), and a signal upper than (lower than) a slice level is outputted. Black (white) as the information of the pre-scanning line written in a buffer memory 23 is rewritten into white (black) at that time. When both the picture elements P2i and P1i are white or black, the output from the amplifier 18 displays a zero level, and the memory 23 is not rewritten.