PURPOSE: To prevent a device automatically from being affected by DC deviation, by detecting the DC deviation of specified quantity or more to connect an AC power source to a load directly, and by stopping the output of control signal from a control circuit at the same time.

CONSTITUTION: The output voltage of a CVCF circuit 3 is inputted to a low pass filter 91 to take out DC component only, and the DC component is inputted to an amplifier 93 via an insulation amplifying circuit 92. DC deviation quantity is determined by the amplifier 93, and the DC deviation quantity is compared with the specified quantity by a comparator 94. When the DC deviation quantity increases and exceeds the specified quantity, then an AND gate 10 falls into a non-logic state, with no output of control signal, and the CVCF circuit is interrupted. At the same time, an AC power source is directly connected to a load by a changeover command circuit 11.