PURPOSE: To obtain accurate shading correcting even when A/D conversions having small resolutions are used and, at the same time, to reduce cost, by previously processing shading characteristics of a reader into signals representing differences from referential set values.

CONSTITUTION: Before reading an original to be transmitted, a referential white mark is read and encoded data of the shading characteristics of a reader is stored in a memory 14. When the orignal is read, encoded data is read out from the referential mark stored in the memory 14 in advance, and the readout data in added to a referential set value after the data is D/A-converted. In this way, shading waveforms are reproduced, and a threshold level S10 are produced from the values. By comparing the threshold level S10 with a video signal S4 of the original by a comparator 16, a binary coded signal of the video signal is obtained. Therefore, accurate binary coded output which is not influenced from the shading characteristic is obtained.