A memory device which can reduce power consumption and a driving method thereof are disclosed. In a memory element including an inverter and the like, a capacitor for holding data and a capacitor switching element for controlling store and release of charge in the capacitor are provided. The capacitor switching element is designed so that the off-state current is sufficiently low. Therefore, even when power supply of the inverter is stopped after charge corresponding to data is stored in the capacitor, data can be held for a long period of time. In order to return data, potentials of output and input terminals of the inverter are set to a precharge potential, then charge in the capacitor is released, and power is supplied to the inverter. A switching element for supplying the precharge potential may be provided.