A method for fabricating a semiconductor device according to the present invention includes the steps of: a) forming an insulator film having Si—H bonds; b) forming a resist mask over a selected region of the insulator film; c) etching part of the insulator film that is not covered with the resist mask, thereby forming a recess in the insulator film; and d) removing the resist mask. The step d) includes the step of e) ashing the resist mask by using plasma produced from a gas comprising water vapor as a main component.