A trench is formed at an exposed portion of a semiconductor substrate by performing a dry etching process with a hard mask of silicon oxide film serving as an etching mask in a dry etching device. At this time, a mixed gas of tetrafluoromethane (CF4), a hydrogen bromide gas (HBr), and a chlorine gas (Cl2) is used as an etching gas. The dry etching process is performed under the condition that a flow rate ratio is more than 0 and less than 0.04, the flow rate ratio being a value obtained by dividing a flow rate NF by a flow rate TF, the flow rate NF being a flow rate obtained by dividing a flow rate of the tetrafluoromethane by the number of fluorine atoms bonded to one molecule of the tetrafluoromethane, the flow rate TF being a total flow rate of the hydrogen bromide gas and the chlorine gas.