PURPOSE: To suppress voltage fluctuation when load chancres suddenly without increasing the gain of an instantaneous value control system.

CONSTITUTION: An output voltage V0 is fed back from the output circuit of a PWM inverter bridge 1 to a control block 3 and then a signal wave is input to a PWM drive circuit 2 via a selection switch 4 provided at the output circuit of the control block 3. A mode selection switch 5 for inputting an error voltage signal(e)which is the difference between the output voltage V0 and a target voltage signal Vref and its differentiated value is provided and an abnormal fluctuation of output voltage is detected by comparing de/dt and a setting value G to change the signal wave output from the selection switch 4, thus controlling output voltage fluctuation.