PURPOSE: To obtain a novel phosphinate ester that is useful as a starting material for producing an antiviral agent and can eliminate the protecting group under mild basic conditions by reacting phosphinic acid with an alcohol or a chloroformic ester.

CONSTITUTION: This novel phosphinate ester is represented by formula I (R1 is a 1-10C aliphatic group; R2 is a 1-10C aliphatic group, a 6-10C aromatic group; R3 is a 1-8C aliphatic group, a 3-8C alicyclic group, a 6-15C aromatic group and the like where in the case that R1 and R2 are each methyl, R3 is not ethyl; in the case that R1 is methyl and the R3 is ethyl, R2 is not p-aminophenyl), and produces a product that can eliminate the protecting group under mild basic conditions and is useful as a starting substance for producing antiviral action. This compound is prepared by esterification of phosphinic acid represented by formula II with an alcohol of the formula: R3OH or with a chloroformic ester represented by the formula: ClCOOR3.