In an apparatus, a first drive unit drives, in a first range of a voltage utilization factor, a switching member to thereby control an output voltage of the power converter to be matched with a command voltage. A second drive unit drives, in a second range of the voltage utilization factor, the switching member to thereby generate a value of a controlled variable of a rotary machine. The second range of the voltage utilization factor is higher than the first range thereof. An estimating unit estimates, during the switching member being driven by the second drive unit, a value of a parameter associated with the output voltage of the power converter. The estimated value is required for the first drive unit to generate the value of the controlled variable generated by the second drive unit.