An optical disk for recording/reproducing information tends to have fine irregularities on its substrate surface that cause an increase in noise, which irregularities are formed during the process of forming the recording layer, which is capable of changing physically or chemically in response to irradiation of laser light. This problem can deteriorate the recording and reproduction characteristics and bring about a defect on a life test or on the storage ability on the disk. To solve this problem, a method of manufacturing an optical disk having a pits-and-lands pattern employs an organic material which is modified by ultraviolet light irradiation. The transmission of the substrate is not more than 50% at one wavelength in a wavelength region from 300 to 375 nm. As a result of the ultraviolet ray irradiation, the surface of an optical disk substrate is smoothed so as to effect a reduction of the substrate noise and an improvement of the recording and reproduction characteristics. Moreover, adhesiveness between the substrate and another layer formed thereon is improved, so that the number of generated defects is reduced.