PURPOSE: To obtain a low birefringent optical disc board by eliminating inequalty in a temperature distribution on the surface of a cavity at the time of injection of resin, by a method wherein the surface of a cavity is heated by providing a temperature gradient where the surface becomes gradually high toward an inner circumferential part from an outer circumferential part by breaking a mold.

CONSTITUTION: A high-frequency induction coil sheet 20 is constituted, for example, of a holding board 21 made of rectangular phenolic resin, a disk 22 made of the phenolic resin provided on one end part of both faces of the holding board 21 and a high-frequency induction coil 23 which is arranged on the disk 22 and in a spiral state and the high-frequency induction coil 23 is connected by a connecting device 25 with the other end part. A groove part 24 in the spiral state where the high-frequency induction coil 23 is arranged is formed in the disks 22 each and a depth of the groove part 24 is made shallow gradually toward an inner circumferential part from an outer circumferential part of the disk 22. When the high-frequency induction coil 23 is arranged along the groove 24, a position of the high-frequency coil 23 becomes high gradually toward the inner circumference from the outer circumference. With this constitution, it becomes that the surface of a cavity 7 is heated so as to negate influence from a heating medium flow path 17 for cooling of a sprue 9.