The light effect on the developing reaction was investigated by electrochemical rotating ring-disk electrode method. It was found that the silver halide formed electrochemically in the dark contained the latent fog specks, some of which would be decreased by the oxidation with oxygen dissolved. In this condition light irradiation caused to increase the developing rate. On the contrary, in the absence of oxygen, light effect was different, resulting in the slow development. This fact can be explained by the bleaching effect of latent fog specks.