To provide a production method of a (poly)alkylene glycol chain-containing polymer which can be preferably used in various applications such as an admixture for cement and which can exhibit extremely high dispersibility to a cement composition. The unsaturated (poly)alkylene glycol ether monomer of the present invention is an unsaturated (poly)alkylene glycol ether monomer production method for producing an unsaturated (poly)alkylene glycol ether monomer by addition reaction of an alkylene oxide on an unsaturated alcohol, wherein the production method includes a step of carrying out addition reaction under condition of existence of 0.01 to 25 parts by mass of an unsaturated (poly)alkylene glycol diether monomer to 100 parts by mass of the unsaturated alcohol, and a (poly)alkylene glycol chain-containing polymer production method, wherein a monomer component including an unsaturated (poly)alkylene glycol ether monomer produced by the production method.