We propose a new lithography technique called “thermal lithography” for patterning fine structures far beyond the diffraction limit. A focused laser spot was used to produce a spatially confined hot area in a photoresist film. This technique enabled us to succeed in fabricating lines and dots with 100 nm dimensions in the photoresist film. The dimensions of the produced patterns correspond to about one fifth of the diffraction limit of 530 nm defined by our optical setup. We will apply this technique to the mastering process for optical ROM disks as a low-cost lithography technique.