PURPOSE: To prevent deterioration due to air oxidation, increase of fog density, deterioration of color reproduction performance, and deterioration of color development density and the like at the time of intermittent use by electrolyzing an aqueous medium containing a composition of a specified color developing agent precursor.

CONSTITUTION: The aqueous medium containing the composition of the color developing agent precursor is electrolyzed to form or to release the color developing agent on an electrode and this precursor is represented mainly by formulae I-III in which each of R1-R6 is an H atom or a substituent; R7 is an alkyl group; R8 is a substituent; X is a group to be allowed to release an NH2 group by the electrolysis; (m) is 0, 1, 2, or 3; each of R9-R12 is an H atom or a substituent; R13 is a substituent; each of R14-R21 is an H atom or a substituent; R22 is a substituent; and (p) is 0, 1, 2, 3, or 4.