Gate insulating films 12A and 12B of different thickness are formed in element openings 16a and 16b in the isolation film 16 of a wafer 10. The gate insulating film 12B is the thinnest gate insulating film. A dummy insulating film having the same thickness as the thinnest gate insulating film 12B is formed in wafer periphery area WP. Gate electrodes 20A and 20B are formed on the gate insulating films 12A and 12B, and thereafter an insulating film is deposited on the wafer surface. The deposited insulating film is dry-etched to form side wall spacers 22a to 22d on side walls of the gate electrodes 20A and 20B. During dry etching, the time when the semiconductor surfaces are exposed in the element opening 16b and area WP is detected as an etching end point by a change in the emission spectrum intensity of etching byproducts.