PURPOSE: To eliminate or extract a synchronizing frequency component in high speed with high accuracy by constituting a control system input detector with a synchronizing frequency generator comprising a PLL and a V/F circuit etc., and a frequency control filter comprising a switched capacitor filter, etc.

CONSTITUTION: A signal fi to be measured outputted from an inverter 13 is fed to the frequency control filter 11 and a prescribed frequency component is eliminated (or extracted) based on a filter clock fc from the variable frequency oscillator 16 fed to the frequency control filter 11. When an output voltage and frequency of the inverter 13 are changed, the oscillating frequency fc of the variable frequency oscillator 16 is controlled by a control circuit (not shown in figure) to which an output of the frequency control filter 11 is fed, the time constant of the frequency control filter 11 is changed to control the inverter 13 with the output of an inverter gate pulse generating circuit 17.