This invention provides a triblock copolymer useful as a compatibilizer for biodegradable polymers, such as polylactic acid, and general-purpose polymers.Namely, this is a triblock copolymer represented by the below general formula (i), wherein each R1 is independently selected from the group consisting of H, —CH3, —C2H5, and —CH2CH(CH3)2, n is an integer of 10 to 1000, and A represents a polyester block obtained by subjecting one or more cyclic esters selected from the group consisting of lactides, glycolides and lactones, to ring-opening polymerization.