An optical disk according to the present invention has a guide groove formed with a width which is not less than 80 nm and not more than 170 nm in terms of a half-value width. A depth of the guide groove is formed within a range in which (n.sub.1-n.sub.2)d.sub.1+n.sub.2d.sub.2=(2m-1).lamda./4 is .+-.15%, wherein n.sub.1 is a refractive index of a substrate, n.sub.2 is a refractive index of a recording film, .lamda. is a wavelength of light beams used for recording and reproduction of information, d.sub.1 is a depth of the guide groove, d.sub.2 is a depth of the recording film positioned on the guide groove, and m is an integer (m=1, 2, 3, 4 . . . ).