An aliphatic polyester polyether copolymer is provided having an aliphatic polyester moiety comprising 35 to 50 molar % of an aliphatic diol unit, 35 to 50 molar % of an aliphatic dicarboxylic acid unit and 0 to 30 molar % of an aliphatic oxycarboxylic acid unit and a polyether moiety, wherein .eta..sub.sp/C is not less than 1.40 and the ratio by weight of the polyether moiety to the aliphatic polyester polyether copolymer is 0.1 to 90% by weight.