A diagnostic apparatus for a multicylinder internal combustion engine includes: an EGR portion capable of executing an individual-cylinder EGR in which EGR gas is distributed and supplied individually to cylinders of the engine; a knock detection portion that detects a knock index value that represents degree of knocking, separately for each of the cylinders; an abnormality detection portion that determines presence or absence of a variation abnormality of air/fuel ratios of the cylinders during execution of the individual-cylinder EGR, and that pinpoints an abnormal cylinder after determining that the variation abnormality is present, and that calculates an imbalance index value that represents the degree of variation regarding the abnormal cylinder; and an abnormality location pinpointing portion that pinpoints an abnormality location in the abnormal cylinder based on the imbalance index value and the knock index value of the abnormal cylinder.