PURPOSE: To utilize regenerative electric power, produced in a crane motor, effectively by a method wherein the title controller is provided with a flywheel controlling inverter for controlling a flywheel motor and a regenerative control unit outputting a commanding signal based on a DC voltage for a crane motor controlling inverter.

CONSTITUTION: A flywheel controlling inverter 16 for controlling a flywheel motor IM3 and a regenerative control unit 17 are provided as a means for absorbing and accumulating regenerative electric power generated by crane motors IM1, IM2. When a regenerative control unit 17 detects the increase of a DC voltage for crane motor controlling inverters 13, 14, the regenerative control unit 17 outputs a commanding signal to the flywheel controlling inverter 16 so as to increase the rotating speed of the flywheel motor IM3. A regenerative electric power, corresponding to the increased amount of the rotating speed of the flywheel motor IM3, is converted into the form of an inertia energy and is accumulated. According to this method, the increase of a DC voltage in the input side of the crane motor controlling inverter 16 can be restrained.