forming first and second insulating films on first and second surface regions of a semiconductor substrate respectively;

forming an n-type semiconductor film on the first insulating film and forming a p-type semiconductor film on the second insulating film;

forming a first protective film on the n-type and the p-type semiconductor films;

forming a second protective film of a material on the first protective film, the material being different from that of the first protective film;

removing the second protective film partially to form first and second patterned films respectively above the first and the second surface regions;

introducing impurities into the second patterned film;

oxidizing the first patterned film and the second patterned film containing the introduced impurities to form an oxide film;

etching the oxide film selectively to slim the second patterned film thinner than the first patterned film;

etching the first protective film using the first and the second patterned films as a mask to form third and fourth patterned films, after the slimming;

etching the n-type and the p-type semiconductor films using the third and fourth patterned films as a mask to form first and second gate electrodes, respectively;

implanting n-type impurities into the first surface region using the first gate electrode as a mask and implanting p-type impurities into the second surface region using the second gate electrode as a mask, in order to form source regions and drain regions in the first and the second surface regions respectively.