An optical disk substrate made of a non-crystalline thermoplastic resin comprising from 80 to 100% by weight of a vinyl cyclohexane polymer (A) containing at least 80% by weight of a vinyl cyclohexane component in the molecular chain and from 0 to 20% by weight of a vinyl aromatic polymer (B) and having a light transmittance of at least 85%, a water absorbance of at most 0.1% by weight and a double refraction of at most 50 nm.
