PURPOSE: To correct a factor for controlling toner concn., to prevent abrupt deterioration of tone concn. when the toner is consumed in high use rate, and to enable its rapid restoration by changing the interval of instruction of toner feed in accordance with the frequency of use of a copy density selector.

CONSTITUTION: CPU4 calculates the sampling interval T by using the following equation. T=n-lm, from the information of the frequency l of copying executed in the mode of a "dark" button 10 being selected where n is a standard value expressed by the frequency of copying, and m is a proportion constant. The CPU 4 controls a scanning system 11 so as to execute batchwise development of a copy sample 6. When theuse frequency of the button 10, since T decreases, too, the abrupt deterioration of toner concn. can be prevented, and its restoration can be elevated in speed.