To supply the power of a power source to a polyphase power converter and to precisely detect power current, without installing a private current sensor when a polyphase motor is driven with the output.

At least two phase currents, iU and iV in three phase current outputted from a three phase inverter 13 as a polyphpase power converter are detected by phase current sensors 14U and 14V. They are inputted, to an arithmetic processing part 21. Thus, the arithmetic processing part 21 calculates remaining phase current iW based on phase currents iU and iV. Thus, a value obtained by individually multiplying phase currents iU, iV and iW by duty ratios DU, DV and DW obtained by dividing on time UON, VON and WON in a pulse width modulation processing for forming control signals CSU, CSV and CSW which drive the three phase inverter 13 by a pulse width modulation period T are added. Then, the power current in of a DC power source 12 is detected.