The reliability of the power supply unit which supplies a different voltage to the microcomputer with two or more power supplies is improved. The power supply unit is composed of first regulator 2, at least one second regulator 4 which generates the voltage lower than that of the first regulator 2, means for detecting the output voltage of the first regulator 2, and means for stopping the second regulator 4 when the output voltage V2 of the first regulator 2 drops lower than the first fixed voltage.