A method of stably and correctly evaluating impurities distribution under a gate of a semiconductor device without damaging a silicon substrate is disclosed. According to the evaluation method, a gate electrode made of a silicon containing material is removed without removing a gate insulating film by contacting pyrolysis hydrogen generated by pyrolysis to the semiconductor device that includes the gate electrode arranged on a semiconductor substrate through a gate insulating film, and a source electrode and a drain electrode formed on the semiconductor substrate on corresponding sides of the gate electrode. Further, a processed form of the gate is evaluated by observing a form of the gate insulating film that remains on the semiconductor substrate, the gate insulating film that remains on the semiconductor substrate is removed by a wet process, and the impurities distribution under the gate is measured and evaluated.