PURPOSE: To perform parallel-in and parallel-out into the power system with the constant-voltage condition being maintained without generating a momentary power failure by finding a detection voltage equivalent to a current voltage by subtracting from the output voltage of a voltage-type self-excited inverter a voltage drop generated by the fact that load current flows into a reactor located on the output side of the voltage-type self-excited inverter.

CONSTITUTION: In a constant-voltage control circuit, an arithmetic circuit is so constructed that a detection voltage VOD of a proportional-plus-integral adjustor 11 may be determined by subtracting a voltage drop to be generated on a reactor 3 due to load current IL from an absolute value of an output voltage V1 of an inverter 1. By this method, the fluctuation of system voltage VS during independent operation is detected by a change in phase information and a change in an effective and an ineffective component of the load current IL of an independent load 5 and the constant- voltage control condition is maintained. And, the constant voltage control circuit of an inverter for independent and linkage operation is obtained wherein parallel-in or parallel-out into the system can be performed without generating a momentary power failure. In addition, since there is no necessity of a constant-current control circuit and a switching device, the structure of the equipment can be simplified.