A power supply unit for supplying power to a processor is disclosed. The power supply unit includes a smoothing capacitor, a controller, and an arithmetic circuit. The controller controls an output voltage according to each power state of a processor operating in a transition state where a power state of the processor transitions between an active state and a predetermined sleeping state at a predetermined transition frequency. The arithmetic circuit determines a transition stop condition based on power consumption of the processor and charging loss generated at the smoothing capacitor during the transition of the active state, and outputs a transition stop signal to stop transition to the predetermined sleeping state.