PURPOSE: To obtain an optical disk having improved corrosion resistance and adhesion property to a base body without decreasing reflectance of a metal reflecting layer.

CONSTITUTION: A base layer 2 having at least 30-400 thickness and comprising yttrium sialon (Y-Si-Al-O-N) having <2.2 refractive index and a metal reflecting layer 3 essentially comprising Al are successively formed on a transparent resin base body 1. Thereby, the reflectance of the metal reflecting layer 3 can be maintained to ≥70 %. Corrosion resistance of the metal reflecting layer 3 and adhesion property to the base body can be improved by the base layer 2 compared to a conventional disk.