A cement dispersion agent contains a water-soluble vinyl copolymer having numerical average molecular weight of 2000-15000 which is obtainable by copolymerization of at least three kinds of monomers shown by Formulas A, B and C given below at copolymerization ratio of 10-50/3-25/80-40 by weight: ##STR1## where R.sup.1, R.sup.2, and R.sup.3 are H or CH.sub.3, R.sup.4 is alkyl group with 1-3 carbon atoms, M.sup.1 and M.sup.2 are alkali metal, alkali earth metal, ammonium or organic amine, and n is an integer in the range 5-25. Such a dispersion agent is produced by aqueous solution polymerization of monomers of these kinds at a weight ratio within the range given above in the presence of a radical polymerization initiating agent. Fluidizing property can be provided to a hydraulic cement composition like concrete, cement paste and mortar by adding an agent of this invention by 0.01-0.2 wt % by solid cement component.