To provide a highly reliable semiconductor device exhibiting stable electrical characteristics. To fabricate a highly reliable semiconductor device. Included are an oxide semiconductor stack in which a first to a third oxide semiconductor layers are stacked, a source and a drain electrode layers contacting the oxide semiconductor stack, a gate electrode layer overlapping with the oxide semiconductor layer with a gate insulating layer provided therebetween, and a first and a second oxide insulating layers between which the oxide semiconductor stack is sandwiched. The first to the third oxide semiconductor layers each contain indium, gallium, and zinc. The proportion of indium in the second oxide semiconductor layer is higher than that in each of the first and the third oxide semiconductor layers. The first and the third oxide semiconductor layers are each an amorphous semiconductor film. The second oxide semiconductor layer is a crystalline semiconductor film.