PURPOSE: To report simply and easily that the quantity of a residual developer recovered into a storage part reaches a prescribed value, by reporting that the stored value of the replenished frequencies of developer reaches a prescribed value.

CONSTITUTION: When a density detecting sensor 46 detects a toner density lower than a prescribed value, the sensor 46 outputs a high-level signal to a control circuit 20, and the circuit 20 drives a replenishing device 45 to replenish the toner. If the density is not raised to a set value or higher for a certain time, the circuit 20 judges that the toner does not exist in a hopper 44, and the circuit 20 drives a warning display lamp 21. Each time the toner is charged into the hopper 44, the circuit 20 adds "1" to the contents of a memory device 22. The circuit 20 discriminates whether these contents reach a prescribed value (m) or not; and when they reach the value (m), the circuit 20 lights a maintenance and inspection display lamp 24. By this lamp 24, the maintenance and inspection (replacement) time of a filter bag 13 is detected.