Provided is a fabrication method of a semiconductor device having an improved production yield.An insulating film for forming sidewall insulating films of a gate electrode is deposited on the main surface of a semiconductor wafer and then, subjected to the treatment for equalizing the film thickness distribution. In this treatment, the semiconductor wafer is fixed onto a spin stage of an etching apparatus and rotated; and an etchant is supplied from an etchant nozzle to the main surface of the rotating semiconductor wafer while moving thereabove the etchant nozzle from the peripheral side to the central side on the main surface of the semiconductor wafer. The moving speed of the etchant nozzle is controlled, depending on the thickness distribution of the insulating film and is made lower in a region where a change rate of the thickness of the insulating film in a radial direction of the semiconductor wafer is large than in a region where the change rate is small.