supply history memory the history a total value and the history b total value are set to zero and the sensor output value immediately after stating the developing unit operation is stored in all the area for 4 seconds in the sensor output memory 25 subsequent to this initialization operation the control operation is started first of all the value of the toner concentration sensor the average of values sampled at an interval of 10 ms as previously described and a rotation stop status are read in based on a sampling period at an interval of one second and written as the latest data in the sensor output memory 25 and the history memory 24 respectively then a difference p between the latest value of the toner concentration sensor and the preset reference value t 0 data the history a total value l 1 the history b total value l 2 and a difference d between the oldest value and the latest value of the sensor output memory 25 are obtained based on information written into the respective memories thereafter the values of p l 1 l 2 and d are multiplied by the corresponding preset sensitivity values p 0 a 0 b 0 and d 0 and summed whereby the current control value f p 0 p a 0 l 1 b 0 l 2 d 0 d is decided in this embodiment the sensitivities p 0 a 0 b 0 and d 0 are set to 1 u2212 0 5 0 25 and 1 respectively when this control value f is positive the.