A method of diagnosing an abnormality of sensorless control of a motor for an air compressor, the method comprising: estimating, by a controller, a speed of a motor of the air compressor in real time and beginning when the motor starts; starting the motor; the method being characterized by: determining, by the controller, whether or not a preset condition for activating a function of diagnosing abnormality of the sensorless control is satisfied, from a slope of the estimated speed of the motor, whereby the preset condition for activating the function of diagnosing the abnormality of the sensorless control is a condition that the slope of the estimated speed of the motor is at or above a preset reference value for diagnosing the abnormality of the sensorless control; performing, by the controller and in a case where the preset condition is satisfied, a control operation for decreasing an amount of electric current which is applied to the motor through an inverter based on an electric current command value; computing, by the controller and in a case where the control operation for decreasing an amount of electric current is performed, an amount of decrease in the slope of the speed of the motor; and determining, by the controller, whether or not the sensorless control fails by comparing the computed amount of the decrease in the slope of the speed of the motor against a preset value for confirming diagnosis of the abnormality of the sensorless control. estimating, by a controller, a speed of a motor of the air compressor in real time and beginning when the motor starts; starting the motor; the method being characterized by: determining, by the controller, whether or not a preset condition for activating a function of diagnosing abnormality of the sensorless control is satisfied, from a slope of the estimated speed of the motor, whereby the preset condition for activating the function of diagnosing the abnormality of the sensorless control is a condition that the slope of the estimated speed of the motor is at or above a preset reference value for diagnosing the abnormality of the sensorless control; performing, by the controller and in a case where the preset condition is satisfied, a control operation for decreasing an amount of electric current which is applied to the motor through an inverter based on an electric current command value; computing, by the controller and in a case where the control operation for decreasing an amount of electric current is performed, an amount of decrease in the slope of the speed of the motor; and determining, by the controller, whether or not the sensorless control fails by comparing the computed amount of the decrease in the slope of the speed of the motor against a preset value for confirming diagnosis of the abnormality of the sensorless control. determining, by the controller, whether or not a preset condition for activating a function of diagnosing abnormality of the sensorless control is satisfied, from a slope of the estimated speed of the motor, whereby the preset condition for activating the function of diagnosing the abnormality of the sensorless control is a condition that the slope of the estimated speed of the motor is at or above a preset reference value for diagnosing the abnormality of the sensorless control; performing, by the controller and in a case where the preset condition is satisfied, a control operation for decreasing an amount of electric current which is applied to the motor through an inverter based on an electric current command value; computing, by the controller and in a case where the control operation for decreasing an amount of electric current is performed, an amount of decrease in the slope of the speed of the motor; and determining, by the controller, whether or not the sensorless control fails by comparing the computed amount of the decrease in the slope of the speed of the motor against a preset value for confirming diagnosis of the abnormality of the sensorless control.