Phosphorus-containing (poly)alkylene glycol compound and polymer

The present invention provides: a polymer which, when used in a cement composition or the like, exhibits higher-level water-reducing properties, workability and other performances and which is useful in various fields of application, particularly as a cement admixture; and a compound which makes it possible to manufacture such a polymer easily and efficiently and at a low cost. The present invention pertains to: a phosphorus-containing (poly)alkylene glycol compound having a structure wherein the terminal oxygen atom at at least one end of the (poly)alkylene glycol chain is bonded to a phosphorus atom with an organic residue therebetween, with the organic residue being bonded to the phosphorus atom by a phosphorus -carbon bond; and a phosphorus-containing (poly)alkylene glycol polymer which comprises a (poly)alkylene glycol chain and structural units derived from a vinyl monomer and which has a structure wherein the terminal oxygen atom at at least one end of the (poly)alkylene glycol chain is bonded to a phosphorus atom with an organic residue therebetween, with the organic residue being bonded to the phosphorus atom by a phosphorus -carbon bond, while the phosphorus atom is bonded to the main chain comprising the structural units derived from a vinyl monomer.