A power supply unit is arranged between a CPU and a power supply device for supplying power to the CPU. Information necessary in proceeding with a program is evacuated from the CPU to an information holding unit. When a power shutdown factor is generated, a power supply control unit outputs a shutdown request signal to the CPU. The CPU, upon receiving the shutdown request signal, activates a power shutdown microprogram, evacuates the information necessary in proceeding with the program to the information holding unit, and outputs an evacuation completed signal to the power supply control unit after the evacuation is completed. Upon receiving the evacuation completed signal, the power supply control unit outputs a power shutdown control signal to the power supply unit. Upon receiving the power shutdown control signal from the power supply control unit, the power supply unit shuts down power supply to the CPU.