An impurity element imparting one conductivity type is included in a layer close to a gate insulating film of layers with high crystallinity, so that a channel formation region is formed not in a layer with low crystallinity which is formed at the beginning of film formation but in a layer with high crystallinity which is formed later in a microcrystalline semiconductor film. Further, the layer including an impurity element is used as a channel formation region. Furthermore, a layer which does not include an impurity element imparting one conductivity type or a layer which has an impurity element imparting one conductivity type at an extremely lower concentration than other layers, is provided between a pair of semiconductor films including an impurity element functioning as a source region and a drain region and the layer including an impurity element functioning as a channel formation region.