A method of etching a substrate by plasma via a mask having a predetermined pattern at back of a silicon layer of the substrate, a semiconductor device being formed at front of which supported by a support substrate, includes a main etching step in which plasma is generated by supplying a process gas including a mixed gas whose flow ratio of fluorine compound gas, oxygen gas and silicon fluoride gas is 2:1:1.5 or a process gas including a mixed gas in which at least the ratio of one of the oxygen gas and the silicon fluoride gas, using the fluorine compound gas as a standard, is larger than the above ratio, and the substrate is etched by the plasma; and an over etching step in which the substrate is further etched by plasma while applying a high frequency for bias whose frequency is less than or equal to 400 kHz.