A motor drive device including a battery 10; switching elements 15 and 16 which are connected in series with a condenser C2 having a voltage Vdc resulting from an increase action of battery voltage and which are operated in a chopper control; a reactor L2 whose one end is connected with a common connection point of the switching elements 15 and 16; and an inverter 19 for driving a PM motor 20 which is connected between another end of the reactor L2 and a negative-pole terminal of the battery 10. In such a motor drive device, an electrical power W is determined based on the voltage Vdc of positive-side point P of the condenser C2, a current Idc flowing in the reactor L2, and a switching duty d1 of the switching element 15 which satisfies a condition of 0≦d1≦1, i.e., is determined by calculating Vdc·d1·Idc.