A light-sensitive material treating apparatus which is small in size and produces a reduced quantity of waste fluid. A current conduction process is applied to a developing solution using an auxiliary tank communicated with a developing tank and to which a supplementary solution is supplied. The developing solution contains a developing agent which is oxidized to an oxidation state by reaction with silver halide and reduced to a reduction state by electronation. The auxiliary tank is separated into two chambers by a cation-exchange membrane, and a cathode and an anode are provided in respective ones of the chambers arranged opposite to each other with respect to the cation-exchange membrane. A current is applied between the two electrodes. The current conduction time is controlled on the basis of current conduction efficiency corresponding to the time of use.