PURPOSE: To harden efficiently gelatin by using a specified compound which reacts selectively with reactive residues in gelatin.

CONSTITUTION: Gelatin is hardened with a compound represented by formula I, wherein each of R1 and R2 is alkyl, aryl or aralkyl; R1 and R2 may bond to each other to form a heterocyclic ring contg. N; Z is a group of nonmetallic atoms required to form a heterocyclic ring contg. N; X- is an anion and may form an inner salt by bonding to R1, R2 or Z; Y is R3-, R4O- or a group represented by formula II or III; R3 is alkyl or aryl; R4 is alkyl or aralkyl; R5 and R6 are R1 and R2, respectively; A is alkylene or arylene; each of L1 and L2 is -O-, -NR7- or a single bond; and R7 is R1 or R2. When the hardening agent is used, fog, stain and deterioration in photographic properties such as desensitization are not caused, and no unfavorable influence is exerted on the reactivity of a coupler. Hardening proceeds rapidly, post hardening is not carried out, and problems about safely and hygiene are not caused because the hardening agent can be added without requiring any org. solvent.