A first gas including a silicon-containing compound is introduced into a vacuum chamber, to expose a semiconductor substrate placed in the chamber to the first gas atmosphere (silicon processing step). Then the pressure inside the vacuum chamber is reduced to a level lower than the pressure at the time of starting the silicon processing step (depressurizing step). Thereafter, a second gas including a nitrogen-containing compound is introduced into the vacuum chamber, and the semiconductor substrate is irradiated with the second gas plasma (nitrogen plasma step).