PURPOSE: To obtain a resist pattern not deformed even by a postbaking and the following treating processes by applying stepwise heating to a developed positive type photoresist film at a temperature lower than resist deformation beginning temperature and higher than it under light irradiation.

CONSTITUTION: A silicon plate is coated with a positive type photoresist film composed of a novolak type polymer and naphthoquinonediazide as a photodecomposing agent and having a deformation beginning temperature of 135°C, and after prebaking, exposure, and development, the film is exposed to light energy of 0.2J/cm2 emitted from a light source in the wavelength region shorter than 450nm for 30sec on a heater correctly set to a temperature of 90W110°C as the first step. Then, as the second step, the silicon plate is conveyed onto a heater correctly set to a temperature of 150W170°C, and exposed to a light energy of 0.2J/cm2 from the same light source for 30sec. During these courses of stepwise irradiation processes, occurrence of abnormal deformation or the like trouble is not found on the resist film, thus permitting a large number of silicon plates to be processed in succession and the obtained resist films to have rectangular section.