Provided is a method of manufacturing a semiconductor device with which a trench shape having vertical, flat, and smooth side wall surfaces can be formed even at room temperature. A semiconductor substrate is placed on a sample stage which is kept at room temperature in a reaction container. A trench is formed in the semiconductor substrate by plasma etching that uses etching gas including oxygen and sulfur hexafluoride, while controlling the gas ratio of oxygen to sulfur hexafluoride so that the gas ratio is from 70% to 100%.