PURPOSE: To enable remaining life of an electrolytic capacitor to be diagnosed without stopping a power supply device for a long time by detecting charging characteristics when the power supply device is operated without any load and by estimating electrostatic capacity of the electrolytic capacitor.

CONSTITUTION: After a switch 11 is turned on and a switch 10 is turned off by a sequence control circuit 21 and then switching is made to power-feed by a direct-supply commercial power supply 1 without any interruption, a rectifier 3 is stopped, a battery connection switch is turned off, and only an inverter 8 is operated without any load with residual charge of an electrolytic capacitor 5 as an input. Charging characteristics of the electrolytic capacitor 5 at this time are detected by a voltage detector 22, which is fed to a remaining life detector. An electrostatic capacity C is estimated by voltage and charging time of the electrolytic capacitor 5. The electrostatic capacitor C and an electrostatic capacity C0 of initial value are compared and it is diagnosed that life expired when the electrostatic capacity was reduced remarkably.