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In the general formula (3), R$_{5 }$ is selected from the group consisting of hydrogen atom, an alkyl group having 1 to 5 carbon atoms and an alkoxyl group having 1 to 5 carbon atoms, with examples thereof including methyl, ethyl, propyl, isopropyl, butyl, pentyl, methoxy, ethoxy, propoxy and the like. 

Preferred examples of the phosphoric acid group› containing monomer represented by the general formula (3) are as follows. 

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Illustrative examples of the pyrophosphoric acid deriva› tive in which two of the compounds represented by the general formulas (1), (2) and (3) are condensed in the form of -P-0-P- through an oxygen atom are as follows. 

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Among the above phosphoric acid group-contammg monomers, preferred are phosphoric acid group-containing monomers represented by the general formulas (2) and (3), particularly phosphoric acid group-containing monomers represented by the general formula (3) in which one of the esters is an aromatic ester, from a view point of adhesive strength to a tooth. 

A mixture of a plurality of the above phosphoric acid group-containing monomers can be used as required. 

The amount of the phosphoric acid group-containing monomer (A) used in the present invention is 0.5 to 50% by weight, preferably 5 to 50% by weight, more preferably 7 to 40% by weight, based on the total of all the constituent components. If the amount of the monomer is too small, adhesive strength to both enamel and dentin, particularly adhesive strength to enamel, will deteriorate, whereas if the amount is too large, adhesive strength to both enamel and dentin, particularly adhesive strength to dentin, will dete› riorate. 

