PURPOSE: To prevent an AC motor from overload in case of performing regeneration and braking while driving the AC motor with an inverter device.

CONSTITUTION: The quantity of regenerated power PS is read in a microcomputer 33 from ROM 34. A regeneration judging circuit 14 judges whether the AC motor 3 is regenerative or not. If it is in regenerative condition, a DC voltage detector 31 and a DC current detector 32 detect the voltage and the current of DC, respectively, and those are read in the microcomputer 33. Next, it seeks instantaneous regenerative power Pn. It seeks residual overload capacity PA being the value ' subtract the sum of instantaneous regenerated power Pn from the quantity PS of the above regenerated power. When the residual overload capacity PA becomes minus, a regenerating power device 4 is stopped to prevent the AC motor 3 from overload.