The invention provides a composite resin composition comprising aliphatic polyester resin (component A) having aliphatic hydroxycarboxylic acid units, one or more biomass material (component B) selected from the group consisting of cellulose, lignocellulose and starch and unsaturated carboxylic acid or a derivative thereof (component C), wherein the composite resin composition comprises a component obtained by covalently bonding a portion or the whole of component A and component B through component C. On processing into various formed items, the composite resin composition of the invention exhibits excellent tensile strength and other physical properties and processability and simultaneously has good biodegradability, and enable biomass materials of low added value to apply to high added value uses in a large amount.