A control circuit portion in an air compressor finds the rate of decrease of pressure inside a tank portion when a motor is at rest relative to the time based on a detection signal from a pressure sensor. Where the rate of decrease of pressure is greater than a given value, the motor is started without waiting for the pressure inside the tank to decrease to a motor restart pressure value, the pressure inside the tank portion rises to a given motor stop pressure value and the motor is then controlled to come to a stop. Where the rate of decrease of pressure is less than the given value, the motor is started when the pressure inside the tank has decreased to the motor restart pressure value. After the start of the motor, the motor is controlled such that it comes to a stop when the pressure inside the tank portion increases to a given value lower than the motor stop pressure value.