A current sensor detects a bus current flowing to an inverter. An estimation section uses actual currents id and id outputted from a d-q conversion section as initial values, estimates currents flowing in a motor/generator and calculates estimated currents ide and iqe. A UVW conversion section converts the estimated current ide and iqe to three phase currents. A selection section inputs, as currents flowing in each phase of the motor/generator, three out of outputs of the UVW conversion section and the bus current. The bus current is used as the current of one phase, if a voltage vector representing an operation state of the inverter is an effective voltage vector.