An electric motor has a field pole formed by a field current passing through a field winding. A voltage booting converter converts output voltage of a battery and outputs the voltage between a power source line and a grounding line. Field winding is electrically connected onto an electric current channel between battery and power source line and formed so that voltage switched by a switching element is applied to both ends. A controller controls the field current so as to adjust density of magnetic flux between a rotor and a stator by performing switching control on switching element and a switching element connected in parallel to field winding and converts the output voltage of battery into voltage in accordance with a voltage command value.