PURPOSE: To ease restrictions on the format design of tablets of a midium by fitting a paper end detector to each divided zone and starting the processing mode from the division whose initial value setting of white level follower has been completed.

CONSTITUTION: A scanning area Z of the Xth scan is divided into plural divisions, a white level follower is initialyzed in each division Z1∼Z4 independently and processing is started from each division Z1∼Z4 whose initial value setting has been completed. Under said condition, subscanning setting width W2 for securing the number of times of subscanning W1 required for the initial value setting of the white level follower can be shortened in the initial data existing area from the end of a paper, easing restrictions on the format design of tablets of a medium.