To estimate the magnetic pole position by suppressing generation of running torque in a motor due to pulse voltage application, and to further estimate the magnetic pole position by suppressing generation of noise or heat from the motor.

In the cell multiplex inverter, each phase arm that applies a voltage to the U, V, W phase of a PM motor outputs a multistep voltage by connecting the output ends of cell units U1-U3, V1-V3 in multiple series thereby superimposing the outputs of respective cell units. At start-up of the inverter, a pulse voltage Vdc for estimating the magnetic pole position is output only from one cell unit U3 among the cell units, and applied to the armature winding of the PM motor while changing the electric angle phase of the pulse voltage, and the magnetic pole position is estimated from an inductance obtained by a change in a current which flows through the winding of the PM motor by applying the pulse voltage.