A semiconductor device is formed using a material which allows a sufficient reduction in off-state current of a transistor; for example, an oxide semiconductor material, which is a wide-gap semiconductor, is used. When a semiconductor material which allows a sufficient reduction in off-state current of a transistor is used, the semiconductor device can hold data for a long time. Transistors each including an oxide semiconductor in memory cells of the semiconductor device are connected in series; thus, a source electrode of the transistor including an oxide semiconductor in the memory cell and a drain electrode of the transistor including an oxide semiconductor in the adjacent memory cell can be connected to each other. Therefore, the area occupied by the memory cells can be reduced.