An input redundant power supply circuit includes a power converter, a relay switch unit including a first relay circuit coupled to a first power source and a second relay circuit coupled to a second power source, a control unit configured to control the relay switch unit so that the power converter is supplied power by the first or second power source, a high capacity capacitor powered by the power converter and a step-up converter coupled between the power converter and the high capacity capacitor. The step-up converter maintains a voltage across the high capacity capacitor higher than a voltage of the first power source or a voltage of the second power source when the relay switch unit switches from one of the first power source and the second power source to the other of the first power source and the second power source.