A molding resin formed from a block copolymer comprising polyhydroxycarboxylic acid structural units (I), and polyester structural units (II) derived from a dicarboxylic acid and a diol; wherein the weight ratio (I)/(II) between the polyhydroxycarboxylic acid structural units (I) and the polyester structural units (II) is within a range from 95/5 to 10/90; the resin has a microphase separated structure in which either the polyhydroxycarboxylic acid structural units (I) or the polyester structural units (II) forms domains within a matrix formed by the other structural units; the average domain size of the domains is within a range from 0.08 to 5.0 .mu.m; and the weight average molecular weight of the molding resin is within a range from 10,000 to 400,000.