It is an object to reduce the effect of a characteristic of the edge portion of a channel forming region in a semiconductor film, on a transistor characteristic. An island-like semiconductor film is formed over a substrate, and a conductive film forming a gate electrode provided over the island-like semiconductor film with a gate insulating film interposed therebetween, is formed over the semiconductor film. In the semiconductor film, a channel forming region, a first impurity region forming a source or drain region, and a second impurity region are provided. The channel forming region is provided in a region which overlaps with the gate electrode crossing the island-like semiconductor film, the first impurity region is provided so as to be adjacent to the channel forming region, and the second impurity region is provided so as to be adjacent to the channel forming region and the first impurity region. The first impurity region and the second impurity region are provided so as to have different conductivity, and the second impurity region and the channel forming region are made to have different conductivity or to have different concentration of an impurity element contained in the second impurity region and the channel forming region in a case of having the same conductivity.