PURPOSE: To prevent the abnormal rise of a white peak, and to prevent the abnormality of reading density by detecting the abnormality of shading distortion information, and preventing white peak follow-up during an abnormal period.

CONSTITUTION: A CPU 19 controls the whole device in conformity to a program stored in a ROM 20, and simultaneously, it detects the period during which a white level falls abnormally in respect of white information read out of a reference white plate as a white level abnormal fall period detection circuit. Besides, the CPU 19 reads a white peak follow-up period stored in the ROM 20, and transfers it to a white peak follow-up circuit 16, and simultaneously, when the abnormal fall period of the white level is detected as being overlapped in the white peak follow-up period, it excludes the abnormal fall period from the white peak follow-up period as a follow-up period varying setting circuit, and sets newly the white peak follow-up period divided into plural parts.