An object is to provide an n-channel transistor and a p-channel transistor having a preferred structure using an oxide semiconductor. A first source or drain electrode which is electrically connected to a first oxide semiconductor layer and is formed using a stacked-layer structure including a first conductive layer containing a first material and a second conductive layer containing a second material, and a second source or drain electrode which is electrically connected to a second oxide semiconductor layer and is formed using a stacked-layer structure including a third conductive layer containing the first material and a fourth conductive layer containing the second material are included. The first oxide semiconductor layer is in contact with the first conductive layer of the first source or drain electrode, and the second oxide semiconductor layer is in contact with the third and the fourth conductive layers of the second source or drain electrode.