An air compressor comprises a tank portion for reserving a compressed air, a compressed air generating portion for generating the compressed air to be supplied to the tank portion, a motor for driving the compressed air generating portion, and a pressure switch mechanism having a pressure sensor in a part of the tank portion, and controlling the motor to be turned on or off in accordance with an output of the pressure sensor, characterized in that when the time change rate of the pressure within air tank is greater than a predetermined value, the motor is restarted without waiting until the pressure decreases to the switch on point at the time of pressure fall.