A light-blocking layer is formed using a first resist mask, and a base film is formed over the light-blocking layer. A first conductive film, a first insulating film, a semiconductor film, an impurity semiconductor film, and a second conductive film are sequentially formed over the base film, and first etching is performed on the second conductive film, the impurity semiconductor film, the semiconductor film, and the first insulating film using a second resist mask over the second conductive film. Then, second etching in which side-etching is performed is performed on part of the first conductive film to form a gate electrode layer, and source and drain electrode layers, source and drain region layers, and a semiconductor layer are formed using a third resist mask. The first resist mask and the second resist mask are formed using the same photomask. Thus, a thin film transistor is manufactured.