To obtain an inverter controller which enables obtaining reactive power outputs which exceed a rated reactive power, as well as active power outputs exceeding a rated active power.

This controller is provided with a step-down transformer 3 with a variable tap, a DC energy device 7, which is connected with the direct-current side of an inverter device 5 and supplies or consumes active power, and a control circuit 10 which controls the variable tap of the step-down transformer. The control circuit 10 is provided with an operation circuit 11, through which the effective value of system voltage is calculated, a setting circuit 12 through which an effective value command value is set, a deviation calculating circuit 13 through which the deviation between an effective value and its command value is calculated, a tap command valve circuit 14 through which a tap command value corresponding to deviation is output, and a deciding circuit 15 through which it is decided whether to vary the variable tap to output an inverter current exceeding a rated value, a tap command valve circuit 16 through which the tap command value is varied based on the result of the decision, a selecting circuit 17 through which either or the tap command values is selected, and a tap varying circuit 18 through which the variable tap is varied based on the selected tap command value.