An uninterruptible power source includes an eco-mode for supplying power from an AC power source to a load via a contactor. In the eco-mode, when a degree of voltage drop of the AC power source reaches 5%, a switching controller turns on a thyristor switch and then turns off a contactor. When the degree of voltage drop of the AC power source reaches 10%, a power conversion controller is configured to cause an inverter to invert DC power of a battery into AC power, and control the inversion in the inverter to synchronize the AC voltage output from the inverter to the AC voltage supplied from the AC power source. When the inversion in the inverter is performed after turning off contactor, the switching controller is configured to turn off the thyristor switch.