The refractive index of a light transmission layer of an optical disk is set within the range of 1.45 to 1.75, the numerical aperture of a lens emitting laser light which is incident onto the light transmission layer is set to 0.65, and the wavelength range of the laser light is set within the range of 395 to 415 nm. Further, in order that aberrations fall within the range of certain acceptable values, the thickness t of the light transmission layer is set within the range of f(n)-t1.ltoreq.t.ltoreq.f(n)+t2, employing constants t1, t2 determined based on an acceptable value of aberration and function f(n) of the refractive index n.