PURPOSE: To judge abnormality in a total fuel system or in a fuel system of each cylinder regardless of a control system of an internal combustion engine in a multi-cylinder internal combustion engine provided with a learning control device for each cylinder by using each correction value set for each cylinder by a learning means of each cylinder.

CONSTITUTION: In a multi-cylinder internal combustion engine 1, the quantity of intake air is adjusted by a throttle valve 3, and is detected by an air flowmeter 4. The engine speed is detected by a crank angle sensor 5. Further, an O2 sensor 9 for detecting the concentration of oxygen in exhaust gas, namely the air-fuel ratio, is installed on an exhaust pipe collective part of each cylinder. A control unit 6 controls the fuel injection quantity of a fuel injection valve 7 based on respective detection signals, and corrects dispersion of the air fuel ratio in each cylinder. In this case, the control unit 6 judges abnormality in the engine fuel system, according to each correction value set for each cylinder. That is, an average value of the respective correction values is determined, and when the average value deviates from a specified range, abnormality is judged.