A method of etching for forming a groove in a SOI substrate includes a forming step, in which a mixed gas plasma is formed by using a mixed gas of a fluorinate gas and an oxygenic gas, and an applying step, in which a high-frequency bias is intermittently applied to the SOI substrate. In the applying step, the high-frequency bias is a temporally modulated high-frequency electricity. According to the method of etching, a yielding rate and a productivity can be improved.