PURPOSE: To prevent the occurrence of deviation of a picture input position by setting the distance between even and odd sensor chips, which are arranged zigzag, to 1/N where N is the greatest common measure of plural line density groups as an object to be read.

CONSTITUTION: Sensor chips 11 of chips 1 to 4 are arranged, and the distance between light receiving parts 13 of even chips and odd chips is (d) in the original carrying direction. (d) is set to 1/100 (inch) and the read line density is 400dpi. When 400dpi, 300dpi, 200dpi, 240dpi, and 180dpi will be attained, M can be an integer because the greatest common measure is 100. Thus, the line density in the subscanning direction can be read at a free read line density without controlling the timing of a first transfer pulse V1 to a subscanning-direction CCD transfer mechanism.