In a method of monitoring washing water used in a developing process of a photosensitive material, a reference conductivity of the washing water is first calculated in accordance with a equation and based on the conductivity of a processing solution in a bleaching/fixing tank and the conductivity of fresh water for replenishing the washing tank. The conductivity of the washing water in the washing tank is compared with the reference conductivity to determine the degree of contamination of the washing water. Since the conductivity of the processing solution may vary due to evaporation thereof or the like, the reference conductivity is updated at periodic intervals. Accordingly, the degree of contamination can always be determined based on a proper reference conductivity.