PURPOSE: To discharge an electrolytic capacitor positively at the time of inspection of an uninterruptible power supply while reducing power consumption during normal operation.

CONSTITUTION: A discharge resistor 10 is connected with the auxiliary contacts of electromagnetic contactors 11, 12 such that the discharge resistor 10 is connected with an electrolytic capacitor 15 only when both contactors 11, 12 are opened. Since the contactors 11, 12 are normally closed and the auxiliary contacts thereof are opened, the discharge resistor 10 is disconnected from the electrolytic capacitor 15 and thereby consumes no power. At the time of inspection, the contactors 11, 12 are opened and the auxiliary contacts thereof are closed to connect the electrolytic capacitor 15 with the discharge resistor 10 thus discharging the electrolytic capacitor 15.