A discharge device and DC power supply system for preventing erroneous operation of a series regulator. The discharge circuit includes a voltage comparison circuit for comparing input power supply voltage and output power supply voltage of the series regulator. The voltage comparison circuit includes first and second transistors. The collector terminal of the second transistor is connected to the drain terminal of a fourth transistor and to the gate terminal of a fifth transistor. The fifth transistor has a drain terminal, which is connected to the output terminal of the series regulator, and a source terminal, which is grounded. Third and fourth transistors operate when the input power supply voltage is greater than the activation voltage. The drain terminal of the fourth transistor is grounded via a resistor, and the voltage of the fourth transistor is supplied to the gate terminal of the fifth transistor.