PURPOSE: To prevent occurrence of deposit and algae during washing, by adding chlorine dioxide in washing water which is not often refreshed.

CONSTITUTION: After use, an automatic developing unit comes to a stop, and then, deposit and algae increases until the developing unit is again used at the next day, and accordingly, after a stop of the developing unit, a predetermined quantity of chlorine dioxide is added. Alternatively, a predetermined quantity thereof is added to washing water. It is particular effective to add it after the developing unit comes to a stop, and further, if the developing unit rests for a long time, the quantity of chlorine dioxide is adjusted correspondingly. The effective density of chlorine dioxide to be used, which differs in dependence upon a kind of a photosensitive material to be used, is 10 to 1000ppm, preferably 20 to 200ppm.