To prevent the abnormality of birefringence due to local support and deformation etc., of a substrate for an optical disk in annealing by implementing annealing in which the inner and outer peripheral parts of the substrate are covered and supported.

A substrate 1 for a synthetic resin optical disk having an opening at the center is molded by injection molding. The obtained substrate 1 is fixed to a holder 5 for annealing to be annealed. In other words, the peripheral part of the opening of the substrate 1 is held by the ring-shaped projection on the inner circumference of a supporting plate 7 and an inner periphery covering member 8, and the outer periphery which is in contact with the ring-shaped projection of the outer circumference of the supporting plate 7 is covered and supported by an outer periphery covering member 9. A holder 5 to which the substrate 1 is attached is introduced into an annealing apparatus and heated at a prescribed temperature for a prescribed length of time.