PURPOSE: To provide an etching method by which a trench, whose opening dimension is small and deep, is easily formed.

CONSTITUTION: A process where an Al2O3 12 is farmed on a silicon substrate 11, a process where a resist pattern 13 is farmed on the Al2O3 12, a process where the Al2O3 is, while the etching gas whose main component is CHF3/CO is made to be plasma state, selectively etched using the etching gas plasma with the resist pattern 13 used as a mask, and a process where the silicon substrate 11 is, while the etching gas whose main component is HBr is made to be plasma state, selectively etched in an anisotropic manner using the etching gas plasma with the resist pattern 13 used as a mask, are provided.