. . "a reflectance Rc of said disk with respect to light with a wavelength of 380 nm to 430 nm, when said recording layer is in a crystalline state, is lower than a reflectance Ra of said disk when said recording layer is in an amorphous state, and wherein a thickness of said lower dielectric layer is set to not less than ??/(9n) and not more than ??/(3n) where n is a refractive index of said dielectri" . .