A method of treating photographic processing wastes is disclosed. The method comprises (1) mixing a silver-recovery system waste and a development system waste from photographic processing to produce a waste mixture, (2) adding to said waste mixture phosphorus as a nutritive substrate in an amount corresponding to at least 2% based on the COD value of the waste mixture, and (3) subjecting the resulting waste mixture to a bio-oxidation treatment using microflora containing sulfur-oxidizing bacteria. The method enables a continuously treatment of high concentration photographic processing wastes with high efficiency over a long period of time.