An acrylate is polymerized by the photo-decomposition of ##STR1## to make a polymeric intermediate containing dithiocarbamate groups at both its ends, and a hard polymer-forming monomer is polymerized by the photo-decomposition of this dithiocarbamate group-containing polymeric intermediate to form blocks of a polymer of said hard polymer-forming monomer on both sides of said polymeric intermediate, thereby forming an ABA type block copolymer.