As the phosphate ester group-containing unsaturated monomer, there can be mentioned phosphoric acid monoesters and phosphoric acid diesters, which are represented by the following general formula (1): ##STR2## where R1 is a hydrogen atom or a methyl group, R2 is a hydrogen atom, or a lower alkyl group having 1 to 3 carbon atoms which may have a substituent, such as a methyl group and a halomethyl