Disclosed is a manufacturing method of a thin film transistor, which enables the formation of a thin film transistor by using only one photomask. The method includes: over a substrate sequentially forming a first insulating film, a first conductive film, a second insulating film, a semiconductor film, an impurity semiconductor film, and a second conductive film; forming a resist mask thereover using a first photomask; performing a first etching to allow the side surface of the layers including an upper portion of the first insulating film, the first conductive film, the second insulating film, the semiconductor film, the impurity semiconductor film, and the second conductive film to be coplanar to a side surface of the resist mask; and performing a second etching to selectively etch the first conductive film to allow the side surface of the first conductive film is located inside the side surface of the layers.