Prior to starting of first and second synchronous motors, direct-current excitation is performed to pull in rotors of the synchronous motors to a designated position. In the direct-current excitation, a difference between a value of a direct current flowing through the first synchronous motor and a value of a direct current flowing through the second synchronous motor is reduced. For example, d-axis currents and q-axis currents corresponding to three-phase currents flowing through the motors are determined; a d-axis voltage command value for making one of the d-axis currents larger in absolute value equal to a d-axis current command value is determined, and a q-axis voltage command value for making one of the q-axis currents larger in absolute value equal to a q-axis current command value is determined; an inverter is driven using the determined d-axis voltage command value and q-axis voltage command value.