To continuously stably operate an inverter system at a high speed by surely preventing the occurrence of a communication failure even when the fluctuation, waveform distortion, and asymmetry of an AC voltage transiently increase, by providing a tolerance angle controlling the reference value changing means which increases or decreases a tolerance angle control reference value.

An tolerance angle control reference value change circuit 81 inputs a tolerance angle control reference value γ0 and, when the variation of a reverse voltage period γ is transiently large, increase the reference value γ0. A minimum value selection circuit 91 selects a constant tolerance angle control signal from a constant tolerance angle control circuit 73, and inputs the selected control signal to a phase control circuit 92. Therefore, the occurrence of a communication failure can be prevented, because the control circuit 92 performs phase control based on the constant tolerance angle control signal inputted from the selection circuit 91. In addition, an inverter system can be operated continuously stably at a high speed by maintaining the power factor and efficiently of the inverter in a steady state.