There is disclosed herein an optical disk substrate comprising a copolymer produced by polymerizing 50 to 65% by weight of methyl methacrylate (a) units, 20 to 35% by weight of tricyclo[5.2.1.02.6]deca-8-yl methacrylate (b) units, 5 to 25% by weight of cyclohexyl methacrylate (c) units and 1 to 5% by weight of acrylic ester (d) units.
