A semiconductor device includes a substrate and an oxide semiconductor TFT supported by the substrate. The oxide semiconductor TFT includes an oxide semiconductor layer containing In, Ga, and Zn, a gate electrode, a gate insulating layer formed between the gate electrode and the oxide semiconductor layer, and a source electrode and a drain electrode that are in contact with the oxide semiconductor layer. The oxide semiconductor layer has a layered structure that includes a first layer, a second layer, and an intermediate transition layer disposed between the first layer and the second layer, and the first layer is disposed closer to the gate insulating layer side than the second layer. The first layer and the second layer have different compositions, and the intermediate transition layer has a continuously changing composition from the first layer side toward the second layer side.