One of factors that increase the contact resistance at the interface between a first semiconductor layer where a channel is formed and source and drain electrode layers is a film with high electric resistance formed by dust or impurity contamination of a surface of a metal material serving as the source and drain electrode layers. As a solution, a first protective layer and a second protective layer including a second semiconductor having a conductivity that is less than or equal to that of the first semiconductor layer is stacked successively over source and drain electrode layers without exposed to air, the stack of films is used for the source and drain electrode layers.