An apparatus or a method for detecting an abnormality of inter-cylinder air-fuel ratio dispersion in a multi-cylinder internal combustion engine according to the invention is equipped with a catalytic element that purifies exhaust gas of hydrogen, a first air-fuel ratio sensor that detects an air-fuel ratio of exhaust gas that has not passed through the catalytic element, a second air-fuel ratio sensor that detects an air-fuel ratio of exhaust gas that has passed through the catalytic element, a device that detects an abnormality of inter-cylinder air-fuel ratio dispersion on the basis of a divergence state of a detected value of a second air-fuel ratio to a lean side from a detected value of a first air-fuel ratio, and a device that forcibly reduces fuel injection amounts in cylinders individually when an abnormality on dispersion is detected by the abnormality detector, detects a divergence state of the detected value of the second air-fuel ratio to the lean side from the detected value of the first air-fuel ratio at this time, and specifies as an abnormal cylinder that one of the cylinders in which a value indicating this divergence state has become smaller than at a time of detection of the abnormality of dispersion.