A full-bridge circuit has serial first arms and serial second arms. Another full-bridge circuit has serial third arms and serial fourth arms. Primary and secondary coils of a transformer are included, respectively, in a part connecting a middle point of the first arms and a middle point of the second arms and a part connecting a middle point of the third arms and a middle point of the fourth arms. A switching phase difference between the first arms and the third arms and a switching phase difference between the second arms and the fourth arms are adjusted, and transmission power between these full-bridge circuits is controlled. Connections of respective electrodes of a power source are inversely connected to respective polarity buses of the other full-bridge circuit. A phase of switching in one of the third and fourth arms other one having a short-circuit failure is inverted.