The invention provides: a cement admixture which has excellent cement dispersibility and can afford high slump retainability; and a cement composition which has excellent cement dispersibility and high slump retainability. The cement admixture comprises a polycarboxylic acid as an effective component, wherein the polycarboxylic acid is obtained by a process including the steps of: charging "a" parts by weight of a polyalkylene glycol (A) and "b" parts by weight of a (meth)acrylic acid monomer (B) into a reactor in the range of {(a/n.sup.1/2)/b}.times.100.ltoreq.200, wherein n is an average molar number of the added oxyalkylene groups in the polyalkylene glycol (A) and is a number of 1 to 300; carrying out an esterification reaction of the resultant mixture in the reactor, thus obtaining a polyalkylene glycol (meth)acrylate (C); and copolymerizing the resultant polyalkylene glycol (meth)acrylate (C) and the (meth)acrylic acid monomer (B), thus obtaining the polycarboxylic acid.