Fluidizing property is provided to a hyraulic cement composition by adding a cement dispersion agent to the composition by 0.01-2 weight % with respect to its solid cement component. The cement dispersion agent contains a water-soluble vinyl copolymer which is obtainable by copolymerization of at least first monomer shown by Formula A given below, second monomer shown by Formula B given below and third monomer shown by Formula C given below at copolymerization rate of 10-50/3-25/80-40 by weight: ##STR1## where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are H or CH.sub.3, R.sup.5 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.