A method for etching is provided in which the etching selectivity of an amorphous semiconductor film to a crystalline semiconductor film is high. Part of a stacked semiconductor film in which an amorphous semiconductor film is provided on a crystalline semiconductor film is etched using a mixed gas of a Br-based gas, a F-based gas, and an oxygen gas, so that part of the crystalline semiconductor film provided in the stacked semiconductor film is exposed. Reduction in the film thickness of the exposed portion can be suppressed by performing the etching in such a manner. Moreover, when etching for forming a back channel portion of a thin film transistor is performed with the method for etching, favorable electric characteristics of the thin film transistor can be obtained. An insulating layer is preferably provided over the thin film transistor.