An air compressor comprising: a tank part for storing compressed air to be supplied to a pneumatic tool, a compressed-air generation part for generating the compressed air and supplying it to the tank part, a drive part with a motor for driving the compressed-air generation part, a pressure sensor for detecting air pressure inside the tank part, and a control circuit part for controlling the motor of the drive part based on a detection signal from the pressure sensor, wherein the control circuit part is adapted to stop operation of the motor when the detected pressure acquired from the detection signal is higher than a maximum set pressure value, and to operate the motor when the detected pressure is lower than a minimum set pressure value lower than the maximum set pressure value, characterised in that the control circuit part is further adapted to operate the motor at a first predetermined revolving speed when the detected pressure drops from the maximum set pressure value to or below a first restart set pressure value defined between the maximum and minimum set pressure values and when a pressure drop rate is greater than a predetermined value, and to operate the motor at a second predetermined revolving speed lower than the first predetermined revolving speed when the detected pressure drops from the maximum set pressure value to or below a second restart set pressure value defined between the maximum and minimum set pressure values and being lower than the first restart set pressure value and when the pressure drop rate is smaller than the predetermined value, and the control circuit part is further adapted to operate the motor in a forced operation where the motor rotates at a set revolving speed until the detected pressure reaches the maximum set pressure value, when the detected pressure has become the same as or lower than the minimum set pressure value. a tank part for storing compressed air to be supplied to a pneumatic tool, a compressed-air generation part for generating the compressed air and supplying it to the tank part, a drive part with a motor for driving the compressed-air generation part, a pressure sensor for detecting air pressure inside the tank part, and a control circuit part for controlling the motor of the drive part based on a detection signal from the pressure sensor, wherein the control circuit part is adapted to stop operation of the motor when the detected pressure acquired from the detection signal is higher than a maximum set pressure value, and to operate the motor when the detected pressure is lower than a minimum set pressure value lower than the maximum set pressure value, characterised in that the control circuit part is further adapted to operate the motor at a first predetermined revolving speed when the detected pressure drops from the maximum set pressure value to or below a first restart set pressure value defined between the maximum and minimum set pressure values and when a pressure drop rate is greater than a predetermined value, and to operate the motor at a second predetermined revolving speed lower than the first predetermined revolving speed when the detected pressure drops from the maximum set pressure value to or below a second restart set pressure value defined between the maximum and minimum set pressure values and being lower than the first restart set pressure value and when the pressure drop rate is smaller than the predetermined value, and the control circuit part is further adapted to operate the motor in a forced operation where the motor rotates at a set revolving speed until the detected pressure reaches the maximum set pressure value, when the detected pressure has become the same as or lower than the minimum set pressure value.