A system power supply apparatus includes a first constant voltage circuit that generates and increases a voltage up to a first constant level when receiving a first control signal. A second constant voltage circuit is provided to generate and increase a voltage up to a second constant level upon receiving a second control signal. The second constant voltage circuit generates and maintains a voltage at a third constant level lower than the second constant level for a prescribed time period upon receiving a third control signal. A control circuit is provided to input the third control signal to the second constant voltage circuit when the system power supply apparatus starts up.