PURPOSE: To read stably lateral minute lines with good reproducibility independently of coarseness of sub-scanning line density by switching a threshold value to binary-code picture information into binary (black/white) picture information to at least ≥2 stages depending on the sub-scanning line density of the reading scanning.

CONSTITUTION: A CCD image sensor 12 photoelectric-converts a picture of an original and outpus an analog picture information signal. Further, the main scanning of the reading scanning is conducted by the CCD image sensor, but the sub- scanning is conducted by feeding the original 10. Binary information when the density of sub-scanning line is dense is stored in addresses 300∼7FF of an ROM 20 and binary picture information when the density of sub-scanning line is coarse is stored in addresses 800∼BFF. Thus, since the binary picture information stored in an address decided by the state of each address input in the range of the addresses 300∼7FF when the sub-scanning line density is dense and in the range of addresses 800∼BFF when the sub-scanning line density is coarse conversely is outputted as an output picture signal F, the output picture signal stable at all times is obtained.