PURPOSE: To prevent fluctuation in the determination of an image forming condition with the use of a determination table so as to enhance the stability of copying operation by allowing sections adjacent to the table to overlap each other.

CONSTITUTION: The copy density is adjusted by a slide volume 21. In a bias setting control part 20, a comparing circuit 24 compares a present read value with a previous level stored in a memory 23. If the present set value is out of a previous level range, determination (for level determination scanning) with the use of a table 25 is carried out. Six stage determination levels are set on the table, and adjacent sections on the table overlap each other. Scanning for determining a level, is successively carried out from a low level, and an initially corresponding section is determined as an exact determination section. The result of the determination is delivered to an output circuit 2, and accordingly, the bias voltage is changed by means of a bias control circuit 11. Further, the result of the determination is stored in the memory 23.