A method of hardening gelatin is described which uses (1) a polymer containing at least 0.01 mol % of a repeating unit represented by formula (I) ##STR1## wherein R represents a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms, or a halogen atom, M represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or an organic base, X represents an alkyl group having from 1 to 6 carbon atoms, an alkoxy group having from 1 to 6 carbon atoms, an alkylamino group having from 1 to 6 carbon atoms, or a halogen atom, m represents 0, 1 or 2, and n represents 1 or 2, (2) a polyvalent alcohol having at least two hydroxy groups, and (3) a hardener having an active vinyl group.The method has advantages in that the rate of hardening the gelatin is fast, the so-called post hardening property is small, and the gelatin composition is hardely influenced by the conditions of temperature and humidity during storage.