The invention provides an inter-cylinder air-fuel ratio imbalance abnormality detection device for a multi-cylinder internal combustion engine, in which when abnormality detection processing of detecting an inter-cylinder air-fuel ratio imbalance abnormality on the basis of revolution speed fluctuations of the multi-cylinder internal combustion engine and ignition timing control for controlling the ignition timing so as to restrict revolution speed fluctuations in the multi-cylinder internal combustion engine are implemented, the implementation of the ignition timing control is restricted when the abnormality detection processing is implemented, and an abnormality detection method to be used in such a device.