In an abnormality detection apparatus and an abnormality detection method for a construction in which each of cylinders is provided with a plurality of fuel injection valves, if it is discerned that a cause of the inter-cylinder variation abnormality exists in one of the fuel injection valves, an air/fuel ratio fluctuation parameter as an index value that represents the degree of the abnormality is calculated by normalizing the air/fuel ratio fluctuation parameter regarding that fuel injection valve on the basis of the injection proportion used when the parameter is measured.