A hardening method for gelatin which comprises using as a hardening agent a compound represented by formula (I) ##STR1## wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 each represents a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted aralkyl group, or a substituted or unsubstituted aryl group; or two of said R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are combined with each other to form a ring (1); or 3 or more of said R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are combined with each other to form a condensed ring (2); X represents a group capable of being released when the compound shown by formula (I) reacts with a nucleophilic reagent; Y.sup..crclbar. represents an anion; or Y.sup..crclbar. is combined with one of X, R.sup.1, R.sup.2, R.sup.3, and R.sup.4, ring (1), or ring (2) to form an anionic portion of an intramolecular salt.