PURPOSE: To prevent the unnecessary fatigue of a developer and to prolong the life of the developer by controlling a stirring means so that it may stop the driving just after judging the effect in the case of judging by a comparison means that toner concentration is high.

CONSTITUTION: An I/O 209 functions to connect a CPU 208 with various external equipments, for example, BK(black) sleeve motor 210 and a BK paddle motor 211, etc., and also, each color toner concentration detecting parts 201 to 204 are connected with an A/D 205, control data which are A/D-converted at a required timing are stored in a RAM 206 so as to be used. Then, in the case of judging that toner replenishment is unnecessary by comparing the toner concentration with a target toner concentration based on the toner concentration detection result, that is, the toner concentration is higher than the target value, the BK paddle motor 211 is turned off just after judging the effect. Thus, the paddle is turned off earlier by the time equivalent to a toner replenishing time.