PURPOSE: To secure a necessary torque for excavation without over loading true driving motor of an excavator and increase the revolution as high as possible to carry out efficient excavation.

CONSTITUTION: A driving controller of an excavator is provided with an ordinary driving circuit 9 driving a motor 7, an inverter circuit 10, a circuit- changeover device 11 selectively changing over both circuits 9, 10 and connecting the motor 7 of excavator, a loading current detector 12 of the motor, a discriminating device 13 of the driving method of the motor, and an inverter speed setter 14. The discriminating device of the driving method of the motor controls the circuit changeover device so as to change the ordinary circuit to the inverter circuit when the load current detected by the current detector is within the preset value and change the inverter circuit to the ordinary circuit 9 when the load current exceeds the preset value. And further, the inverter frequency is raised when the load current is within a preset figure in a specified frequency and the inverter frequency is kept constant when it exceeds the preset figure.