To regulate the slip correction rate for correcting the slip frequency by providing means tar operating a slip correction rate and then operating a second slip frequency based on a first slip frequency and the slip correction rate.

A slip frequency operating means 32 operates a first slip frequency ωs based on a torque component voltage Iq. A slip correction rate operating means 37 operates a slip correction rate which is then employed, along with the first slip frequency ωs, for operating a second slip frequency ωs'. An external signal decision means 38 makes a decision whether the slip correction rate is valid or not based on an external signal inputted from an external signal input means 29. An output frequency operating means 39 operates an output frequency ω1 based on a target frequency ω1' and the second slip frequency ω'.