a motor drive device including a battery 10 ; switching elements 15 and 16 which are connected in series with a condenser c 2 having a voltage vdc resulting from an increase action of battery voltage and which are operated in a chopper control ; a reactor l 2 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 l 2 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 c 2 , a current idc flowing in the reactor l 2 , and a switching duty d 1 of the switching element 15 which satisfies a condition of 0 ≦ d 1 ≦ 1 , i . e ., is determined by calculating vdc · d 1 · idc .