To perform retract operation done at the time of starting a motor while changing parameters automatically, by initially setting starting frequency and starting voltage, determining whether or not motor speed reaches lead-in speed, and controlling the starting frequency or starting voltage.

Retract control is performed so as to make rotor speed meet synchronous speed. Respective initial values of starting frequency and starting voltage are set, and a current limit value and target speed are also set. After the parameters are set, a motor is driven, and a rotational speed deciding part 10 compares motor speed with the lead-in speed corresponding to the starting frequency. If the motor speed is below the lead-in speed, the starting frequency remains fixed, and a voltage control part 11 decides current after starting voltage is raised. By repeating this process, and detecting that the motor speed reached the lead-in speed, it is decided that the lead-in operation is completed.