To mitigate the influence of instantaneous power failure of a bypass power source by keeping ON condition while the switch to supply the bypass power between the bypass power source and load is on.

An uninterruptible power unit 1 stops power supply by the occurrence of abnormality. Moreover, when an ON signal is inputted into the semiconductor element 32 of an AC switch 3, the semiconductor element 32 is turned on, and supplies power through a breaker 5 from a bypass power source 4. Accordingly, switching without a break from the uninterruptible power unit 1 to the bypass power source 4 can be realized. Furthermore, the semiconductor element 32 of an AC switch 3 keeps the ON condition also after turning on of the switch 31 of the AC switch 3. Accordingly, the influence of the instantaneous power failure of the bypass power source can be prevented to its minimum by making the semiconductor element of the AC switch act upon it.