PURPOSE: To prevent an unnecessary power from being supplied to a motor at regeneratively operating time by detecting a supply power from an input current and thereby correcting a slip.

CONSTITUTION: The input current of an inverter 5 is measured by a current detector 15 at regeneratively operating time, and the power supplied to the inverter is detected by a power detector 16. The initial value of a slip S is so set in correcting means in a microcomputer 22 that a regenerative power is not returned absolutely to a power source side, and the slip S is corrected to always operate slightly at power drive side. Thus, the slip at the regeneratively operating time can be set to the optimum value to prevent the unnecessary power from being supplied to the motor at the regeneratively operating time, and a device for processing the regenerative power can be eliminated to simplify a controller.