To simplify a process for power gating.

A semiconductor device has a volatile holding node capable of holding data, a capacitor, and a transistor with a channel formed in an oxide semiconductor layer for selecting whether or not to electrically connect the holding node and the capacitor. The transistor remains on for substantially all the period except the period when the semiconductor device is power-gated. The capacitor thus normally has data corresponding to the data at the holding node, and the data is completely saved from the holding node to the capacitor only when the transistor is turned off. A simple process (turning off the transistor) can therefore complete preparations for power gating.