To make turn on/off of the power supply to a peripheral device possible in an interlocking way with the turning on/off of the power supply to a terminal device connected to an uninterruptible power unit without human intervention nor connecting any special device and, at the same time, to prevent the capacity of the uninterruptible power unit from becoming excessively large.

An uninterruptible power unit is provided with a main plug socket 2 to which a terminal device is connected and a first sub-plug socket 3 to which a peripheral device that requires backup when a power failure occurs is connected, and a second sub-plug socket 4 to which a peripheral device that requires no backup is connected. A power supply control section 7 detects the power supply turn-on/off state of the terminal device 8 connected to the plug socket 2 and controls the power supply to the sub-plug sockets 3 and 4 in an interlocking way with the detected state. Since a main control section 5 outputs a direct current from a battery to the plug socket 2 and sub-plug socket 3 after converting the current into an alternating current, but does not output the alternating current to the sub-plug socket 4, when the power failure occurs, the capacity of the uninterruptible power unit is prevented from becoming larger at the time of calculating the capacity.