A processing solution such as a developer and fixer is maintained at optimum processing ability by conducting electricity through an anion exchange membrane. In FIG. 1 , for example, an anion exchange membrane A1 and an only H.sup.+ and/or OH.sup.- permeable diaphragm M1 are disposed in a tank 11 to define three compartments 11a, 11b, 11c which are charged with electrolyte solution E1, developer Dev, and electrolyte solution E2 and have disposed therein anode 22, cathode 21 and anode 24, respectively. Silver halide photosensitive material is processed by passing it through the developer. Electricity is conducted between anode 22 and cathode 21 during processing and between anode 24 and cathode 21 during quiescent periods.