To make it possible to stably detect a single operation of an inverter constituted in such a manner that the output can be supplied to a system by sending a signal for indicating the inverter being in a single operation when the output voltage or output wavelength of an inverter is compared with a system voltage or system wavelength and both of them coincide with each other.

A shift value is added to a value corresponding to the system voltage or a value corresponding to the system wavelength, and drive signals for inverter 2 are prepared in a drive circuit 6 based on an output wavelength control signal for inverter 2 or an output voltage control signal for inverter 2 obtained from a PLL circuit 5. Output voltages or output wavelength of the inverter 2 output based on the drive signal are respectively compared with the voltage or wavelength of the system 10 in a single operation detection circuit 7 and, if both of them coincide each other, then a single operation detection signal indicating the inverter 2 being in single operation is transmitted from the single operation detection sequence circuit 7. By doing this, the single operation of the inverter 2 can be detected definitively.