A method for manufacturing a semiconductor device, by which a bottom gate thin film transistor that has an improved S value and a channel forming region with a smaller thickness than that of a source region and a drain region can be manufactured in a simple process. An island-like conductive film is formed over a surface of an insulating substrate in a portion corresponding to a channel forming region, and is covered with an insulating film to form a projection portion. After an amorphous semiconductor film is deposited to cover the projection portion, the amorphous semiconductor film is irradiated with laser light so as to be melted and crystallized. Part of the melted semiconductor over the projection portion flows into regions adjacent to both sides of the projection portion, which results in reduction in thickness of the semiconductor film over the projection portion (channel forming region).