A PWM signal is generated without adding any special device by a simple method while imparting a high degree of freedom of modulation ratio and preventing extreme degradation of the efficiency. PWM signal duty creating means (22) creates two basic voltage vectors having a 60-degree phase diffidence and at least one zero vector by a conventional method. PWM signal duty re-distributing means (23) distributes the zero vector generation time ratio to three basic voltage vectors having a 120- degree phase difference between them and including the basic voltage vector present at the rear end in the direction of the inverter rotation out of the two basic voltage vectors having the 60-degree phase difference by using three vectors constituting a zero vector corresponding to the voltage command vector V* and having the same length and a 120-degree phase difference between them, and creates three basic voltage vectors having a 60-degree phase difference between them and a zero vector from the three basic voltage vectors having the 120-degree phase difference between them.