To provide a transistor with favorable electrical characteristics, a transistor with stable electrical characteristics, or a highly integrated semiconductor device. An electrode is provided over an oxide semiconductor layer A, the oxide semiconductor layer A and the electrode are covered with a layer C, and then heat treatment is performed; thus, oxidation of the electrode which is caused in the heat treatment is prevented. For the layer C, for example, an oxide semiconductor can be used. By covering a side surface of the oxide semiconductor layer A where a channel is formed with the layer C and the oxide semiconductor layer B, diffusion of impurities from the side surface of the oxide semiconductor layer A into the oxide semiconductor layer A is prevented.