Aliphatic polyester resin and method for producing same

An aliphatic polyester resin having a .lambda. value representing the magnitude of non-linearity of elongational viscosity of 1.5 to 8.0, said .lambda. value being defined by the following formula (1):

• .lambda.=.lambda..sub.1 /.lambda..sub.0 ( 1)
• wherein .lambda..sub.0 denotes the elongational viscosity at a transition point and .lambda..sub.1 denotes the elongational viscosity when strain becomes twice that of the strain at the transition point, wherein the transition point means the point between a linear region (infinitesimal-deformation region) and a non-linear region (large-deformation region),
• wherein the aliphatic polyester resin is mainly constituted by an aliphatic glycol and an aliphatic dicarboxylic acid or anhydride, the resin being obtained by reacting the glycol and the dicarboxylic acid with at least one polyfunctional compound selected from the group consisting of a tri- or more polyol, a tri- or more oxycarboxylic acid or anhydride and a tri- or more polycarboxylic acid or anhydride, wherein the resin has a weight-average molecular weight (Mw) of at least 20,000 and is provided with long chain branched formation.