The present invention provides a method for manufacturing a highly reliable semiconductor device with a small amount of leakage current. In a method for manufacturing a thin film transistor, etching is conducted using a resist mask to form a back channel portion in the thin film transistor, the resist mask is removed, a part of the back channel is etched to remove etching residue and the like left over the back channel portion, whereby leakage current caused by the residue and the like can be reduced. The etching step of the back channel portion can be conducted by dry etching using non-bias.