PURPOSE: To weaken force to be applied on a gear and to prevent abrasion of the gear by stopping variation of output frequency for a preset time after a preset variation of the frequency when switching is made from stopped state to accelerating operation.

CONSTITUTION: An output frequency stop signal outputting circuit 10 comprises a f judging circuit 101 and a timer circuit 102. Output control of an output frequency stop signal is carried out based on an output frequency command provided from an output frequency determining circuit 7. When switching is made from stopped state to accelerating operation, variation of output frequency is stopped for a preset interval after preset variation of the frequency. Force to be applied on the gear for connecting an induction motor 4 with a load unit is weakened by regulating the variation of the frequency and the preset time interval and thereby abrasion speed of the gear is retarded.