To obtain a prompt decoloring method excellent in decoloring effect, free from secondary contamination by the generation of sludge and others, capable of the reduction of treatment costs, and developing both decoloring effect and photographic properties by treating an azo dye with ozone.

An azo dye given by formula I is decolored by ozone treatment. In the formula, Q represents an acidic nucleus; M represents a hydrogen atom or a cation. The acidic nucleus given by Q is preferably a pyrazolone given by formula II or a pyridine given by formula III. In the formula II, R1 is alkyl, aryl, amino, acylamino, ureido, carboxyl, alkoxycarbonyl or the like; R2 is hydrogen, alkyl, aralkyl, or aryl. In the formula III, R3 is hydrogen, hydroxyl or alkyl; R4 is hydrogen, alkyl, aralkyl or aryl; and R5 is hydrogen, cyano, carboxyl or the like.