A backup circuit that can be fabricated by the standard CMOS process and has a small circuit scale. The backup circuit (10) is disposed between a digital circuit (20) including a storage circuit and a power supply terminal (TIN, TGND) for supplying power to the digital circuit. MOS transistors (MOS1, MOS2) connected in series are disposed between the power supply terminal (TIN, TGND) and a backup capacitor (C1). The MOS transistors (MOS1, MOS2) serve as resistors when the power is normally supplied to the power supply terminal, and as diodes each operating with its backward direction defined as the direction from the digital circuit toward the power supply terminal when the power is cut off.