An inverter that supplies a desired primary voltage (v1) to an induction motor, a current detector that detects a primary current (i1) from the inverter to the induction motor, a voltage designation value calculating unit that generates a voltage designation value (v1*) for generating the voltage v1, an idling-time speed estimating unit that calculates an estimate value (ωr#) of the rotation speed when the induction motor is idling, and a sequence circuit that commands the operations of these units are provided. The idling-time speed estimating unit switches a method for deriving the estimate value on the basis of a current value in the case where a short circuit is generated between windings when the induction motor is idling.