The concentration of halogen ions in a photographic developer is controlled at an almost constant value by determining the concentration of halogen ions in the developer by detecting the change in electric potential caused by the addition of silver ions to the solution, and controlling the electrical current supplied to a halogen ion removing means, such as an electrodialysis or an electrolysis unit having an anion exchange membrane, based on the result of the determination of halogen ions. By this method the amount of complementary developer added to a fatigued developer solution can be greatly reduced.