PURPOSE: To precisely detect the aging degree of a developer to prevent developing errors and to use the developer efficiently and usefully, by measuring the oxidation and reduction potential of the developer to detect the aging degree and indicating it with a meter.

CONSTITUTION: A film 1 to be developed is inserted from an inlet 28 and discharged from an outlet 26 through respective tanks of the developer (a), a fixer (b) and rinsing water (c). The aging degree of the developer is detected by a potential difference generator 4 and the detected value is indicated by a meter 13 through an amplifier 5 and a meter driving amplifier 12. It is also applicable to divide stepwise the scale of the meter 13 with colors such as red, yellow and green. The driving of the meter 13 can be connected also to the working switch of an alarm buzzer or a pilot lamp prepared in another circuit.