A method for producing a phenylphosphonic acid metal salt composition, including reacting a phenylphosphonic acid compound (a) with a metal salt, metal oxide or metal hydroxide (b) that is present in an amount beyond the equivalent, the phenylphosphonic acid metal salt composition containing phenylphosphonic acid metal salt, and a surplus amount of the metal salt, the metal oxide or the surplus metal hydroxide (b). A crystal nucleating agent comprises the phenylphosphonic acid metal salt composition produced by the method.