To provide a method for formation of photoresist patterns having excellent dimensional controllability and resist peelability while maintaining heat resistance and dry etching resistance.

In a lithography stage for transferring mask patterns to positive type chemical amplification type resist applied on a semiconductor substrate, the chemical amplification type resist 1 is applied on the semiconductor substrate 3 and is subjected to exposing and developing to form the resist patterns 1A and thereafter, the chemical amplification type resist is irradiated with radiation having a single peak at a specified wavelength over the entire surface and thereafter, the semiconductor substrate is heated. The full-surface irradiation quantity is ≥2 times the film omission sensitivity of the resist. The refractive index to a wavelength 633 nm after the full-surface irradiation is increased by ≥3% than that before the full-surface irradiation with the radiation, by which the line width of the resist patterns after the full-surface irradiation it reduced by ≥5% than that before the full-surface irradiation.