PURPOSE: To prevent the lowering of fluidity with the lapse of time and to improve the water reducing property by using the copolymer of three kinds of specified monomers as the main components.

CONSTITUTION: 30-90mol% monomer shown by formula I [R1 is H or methyl, and M is H, alkali (earth) metal, -NH4 or amino compds., 10-70mol% monomer shown by formular II and 0.01-10mol% monomer shown by formula III ((n) is 2 to 6, and X is an alcohol residue having ≥2 hydroxyls) are radical- copolymerized in solvent at 0-100°C in the presence of a polymerization initiator and a chain transfer agent to obtain a cement admixture consisting of the copolymer having 1,000-200,000 mol.wt. This cement admixture is used by 0.01-2 pts.wt. for 100 pts.wt. of cement.