Abstract: When a voltage conversion operation is started, a control circuit (30) performs an operation to obtain a voltage command value for each control timing with setting a target voltage obtained based on a torque command value (TR) and a motor rotation number (MRN) as a final value, and controls a boost converter (12) so as to match an output voltage (Vm) with the voltage command value. The control circuit (30) has a prescribed threshold value set to be lower than the target voltage. The control circuit (30) controls the boost converter (12) with setting an absolute value of a rate of change between control timings to a first value until the voltage command value reaches the prescribed threshold value. When the voltage command value becomes at least the prescribed threshold value, the boost converter (12) is controlled with setting the absolute value of the rate of change to a second value smaller than the first value.