A method of manufacturing a semiconductor device forms the semiconductor device in a device region of a semiconductor substrate simultaneously with forming a monitor semiconductor device that includes a gate electrode made of silicon containing material arranged on a gate insulating film in a monitor region of the semiconductor substrate, a source electrode and a drain electrode formed on the semiconductor substrate on corresponding sides of the gate electrode. The gate electrode is removed without removing a gate insulating film by applying pyrolysis hydrogen generated by pyrolysis on the monitor semiconductor device in the monitor region, and the gate insulating film is removed by a wet process. Impurities distribution of a silicon active region appearing after the gate electrode is removed is measured and fed back to a semiconductor manufacturing process.