The present invention relates to a resin composition for foam, which is used as a packing cushioning material, and an object thereof is to provide said composition which has biodegradability and is made from a principal raw material, lactic acid capable of contributing to the protection of global environment. The object can be attained by a resin composition having biodegradability and expandability, comprising a high-molecular weight polylactic acid having a melt viscosity of 0.01 to 50 in terms of a melt index value (MI), characterized in that said resin composition is prepared by compounding 0.1 to 5% by weight of at least one compound, which is selected from a polyisocyanate having an isocyanate group of not less than 2.0 equivalents/mol, an epoxy compound having an epoxy group of more than 2.0 equivalents/mol and an acid anhydride having an anhydrous carboxyl group of more than 2.0 equivalents/mol, with a prepolymer made of lactic acid wherein a molar ratio of a L-isomer to a D-isomer is within arange from 95:5 to 60:40 or from 40:60 to 5:95, and reacting the resulting mixture.