DC backup power supply device and method for diagnosing the same

A reliable uninterruptible DC power supply device. The DC backup power supply device includes an AC/DC converter (2), a DC/DC converter (3), voltage step-up/down choppers (151, 152, 16, 18) and a battery (19) connected to a DC path of a main circuit connected with a load (4) via a switching means (13), and a microcomputer (20). In the device, under control of the microcomputer, the voltage step-up/down choppers are first operated under a condition that the MOS FET (13) was turned OFF for self diagnosis of the backup power supply device. Next, the switching means (13) is turned ON to execute the remaining self diagnosis. The DC backup power supply device can execute its self diagnosis with a lessened likelihood of danger of exerting an adverse effect on the main circuit and also can exhibit a reliable uninterruptible power supply function.