A voltage detector 17 for detecting a voltage VB of a battery 9 as a main power supply and a voltage detector 18 for detecting a voltage VC of an auxiliary power supply 14 are provided. On the basis of an upper limit value IB previously defined as the maximum value of current through the battery 9, a control circuit 6 defines an upper limit of the electric power supplied to the motor 4 as η·VB·IB (η representing the efficiency of the driving circuit 5) when a first output mode is selected in which the auxiliary power supply 14 is not used or defines the upper limit of the electric power supplied to the motor 4 as η·(VB+VC)·IB when a second output mode is selected in which the auxiliary power supply 14 is used as connected in series with the battery 9.