A manufacturing method of a semiconductor device includes the steps of: forming a gate electrode over a substrate; forming a gate insulating film over the gate electrode; forming an oxide semiconductor film; performing heat treatment to form a second oxide semiconductor film after the step of forming the first oxide semiconductor film; forming a first conductive film; forming a first resist mask including regions whose thicknesses are different; etching the second oxide semiconductor film and the first conductive film using the first resist mask to form a third oxide semiconductor film and a second conductive film; reducing the size of the first resist mask to form a second resist mask; selectively etching the second conductive film using the second resist mask to remove a part of the second conductive film so that a source electrode and a drain electrode are formed.