A surface treatment method is provided, wherein the ratio of the etching rate of a lower resist of a multilayer resist film used for forming a fine pattern to that of an inorganic intermediate layer thin film serving as a mask to control the dimension of the pattern, that is, the shoulder selection ratio, is increased in an etching treatment of a semiconductor or the like. In the surface treatment method of a semiconductor, in which an inorganic intermediate film and an upper resist film are laminated on a lower resist film, by using plasma, CO2 containing oxygen as a primary component is added to a gas composed of nitrogen and hydrogen, so that an etching gas is prepared. Consequently, cutting of a shoulder of the inorganic intermediate layer film is reduced and a perpendicular shape is attained.