An inverter includes a voltage command generator generating a voltage command value according to an externally specified voltage value, a PWM signal generator generating a PWM signal according to the voltage command value and frequency command value, and a switching unit generating a three-phase AC power according to the PWM signal. The voltage command generator decreases the voltage command value if the output current increases, to prevent the rotation speed of a prime move from suddenly changing. If the output current exceeds a preset upper current threshold, the voltage command value is clamped at a preset minimum output voltage, thereby securing the minimum output voltage for an increase in the output current.