A cement retarder which comprises a saturated or unsaturated polyester obtained by a reaction of a polycarboxylic acid component comprising a polycarboxylic acid having 2 to 6 carbon atoms in the main chain or its derivative, and a polyol component containing a polyhydric alcohol having 2 to 4 carbon atoms or its condensate, wherein:(a) the saturated polyester has a weight average molecular weight of about 300 to about 50,000; or(b) the unsaturated polyester has a weight average molecular weight of about 500 to about 25,000; or(c) the polyhydric alcohol used to obtain the saturated or unsaturated polyester is a condensate of a C.sub.2-4 alkylene glycol; or(d) the saturated polyester is a polyester obtained by the use of a condensate of a C.sub.2-4 alkylene glycol and has a weight average molecular weight of about 300 to about 50,000; or(e) the unsaturated polyester is a polyester obtained by the use of a condensate of a C.sub.2-4 alkylene glycol and has a weight average molecular weight of about 500 to about 25,000.