Motor loss increment driven by PWM inverter is investigated by experimental results of induction motor in comparison with sinusoidal wave source. PWM inverter has a lot of time harmonic components in its output voltage, though sinusoidal wave source has no harmonics. Since the deviation of motor loss between PWM inverter drive and sinusoidal wave source is considered to be a motor loss increment affected by PWM inverter, it is compared with the inverter circuit loss. As a result, it is founded that motor loss increment is larger than inverter circuit loss in some conditions. In order to decrease the total inverter loss, which includes the inverter circuit loss and the motor loss increment, optimum carrier frequency operation is desirable in no-loaded condition, though the higher carrier frequency is desirable in loaded condition. Therefore, motor loss increment by PWM inverter should be paid attention in designing PWM inverter.