PURPOSE: To extend the flow control range by using output frequency change and voltage control through driving a generator at a plurality of speeds with a pole change motor or the like while using a MG set as a driving source.

CONSTITUTION: In a power apparatus using a MG set 29 having moment of inertia as the driving source of a solenoid pump 8 for transferring a liquid metal or the like, while using a pole change motor 28 having a plurality of speeds as a driving motor of the MG set 29, a slip of the solenoid pump 8 is controlled and a transfer flow rate is adjusted by the change of rotational speed of a generator 14 and output voltage control. Also, a plurality of motors having a single speed, the pole change motor 28 having a plurality of speeds and a motor having a single speed are used as the driving motor of the MG set 29.