PURPOSE: To exactly detect the residual amt. of a toner by detecting whether the housing amt. of a developer detected by a detecting means during the rotation of an agitating device after counting of the prescribed time by a time counting means attains the prescribed lower limit amt. or not and announcing the same when the prescribed lower limit amt. is attained.

CONSTITUTION: The time counting means 16 is started by the start of rotation of the agitating device 52. The housing amt. of the developer 13 in a housing part 50 is detected by the detecting means 14 during the subsequent rotation of the agitating device 52 when the prescribed time is counted. Decision is made whether this housing amt. is below the housing amt. or not. The result of the decision is announced when the housing amt. is below the prescribed amt. The fluctuation in the detection output of the housing amt. by the rising at the start of rotation of the agitating device 52 is averted in such a manner and the housing amt. of the developer 13 of the housing part 50 is decided by the stable detection output after the lapse of the prescribed time. The detec tion error is, therefore, prevented. the residual amt. of the toner is exactly detected in this way.