In an electric vehicle in which a motor is driven using the electric power of a battery, and a PWM controller controls an inverter mounted between the battery and the motor. The frequency of the PWM control means is usually set higher than an audio frequency, in order to decrease the switching noise of the inverter. Motor operational state detecting devices detect when the motor is at low speed and high-load operational state, and if there is a possibility that switching elements of the inverter may be over-heated, the frequency of the PWM controller is decreased by a frequency changing device, thereby preventing damage to the switching elements of the inverter due to the over-heating.