To prevent an inverter from failing and to securely obtain a starting torque by driving a motor based on a frequency and a voltage based on a specific polygonal line pattern of frequency and voltage in starting and linearly decreasing the voltage in deceleration stop.

A control method can be applied by outputting a voltage and a frequency following the V/f pattern of a polygonal line V/f control without limiting to V/f control. Then, when the polygonal line V/f control is used for driving a pump requiring a high starting torque, a high starting torque can be obtained by setting a polygonal line voltage V2 and a polygonal line frequency f2 by the polygonal line V/f pattern. However, in the deceleration stop, a load rate rapidly decreases and reaches nearly unloaded state once being started even in the case of highly viscous fluid, so that an inverter output voltage is decreased linearly, thus securing a large starting torque and at the same time preventing an overcurrent and an abnormal load failure from occurring in the deceleration stop.