An exhaust manifold mounting structure for an internal combustion engine, comprising: a cylinder head in which exhaust port outlets (4a, 4b, 4c, 4d) are arranged in multiple rows and a plurality of the exhaust port outlets are arranged in each row, a fastening seat portion being formed at an end face of the cylinder head, the plurality of exhaust port outlets being arranged at the fastening seat portion; and an exhaust manifold having a plurality of exhaust gas introduction openings (6a, 6b, 6c, 6d) corresponding to the exhaust port outlets, the exhaust manifold including a flange in a shape corresponding to the fastening seat portion, the exhaust manifold being mounted on the fastening seat portion, the exhaust manifold mounting structure is characterized in that: a recess is provided at one of the fastening seat portion of the cylinder head and the flange of the exhaust manifold in correspondence with an area in between any adjacent two of the exhaust port outlets (4a, 4b, 4c, 4d), and a protrusion that is fitted to the recess is provided at the other one of the fastening seat portion of the cylinder head and the flange of the exhaust manifold, and the exhaust manifold is fastened to the cylinder head by placing the flange of the exhaust manifold on the fastening seat portion of the cylinder head in a state where the protrusion is fitted to the recess and wherein the exhaust manifold mounting structure further comprises: a knock-pin provided at a center portion of the fastening seat portion of the cylinder head; and an insertion hole, to which the knock-pin is inserted, and which is provided at a center portion of the flange of the exhaust manifold, wherein the knock-pin is inserted in the insertion hole in a state where the flange of the exhaust manifold is placed on the fastening seat portion of the cylinder head. a cylinder head in which exhaust port outlets (4a, 4b, 4c, 4d) are arranged in multiple rows and a plurality of the exhaust port outlets are arranged in each row, a fastening seat portion being formed at an end face of the cylinder head, the plurality of exhaust port outlets being arranged at the fastening seat portion; and an exhaust manifold having a plurality of exhaust gas introduction openings (6a, 6b, 6c, 6d) corresponding to the exhaust port outlets, the exhaust manifold including a flange in a shape corresponding to the fastening seat portion, the exhaust manifold being mounted on the fastening seat portion, the exhaust manifold mounting structure is characterized in that: a recess is provided at one of the fastening seat portion of the cylinder head and the flange of the exhaust manifold in correspondence with an area in between any adjacent two of the exhaust port outlets (4a, 4b, 4c, 4d), and a protrusion that is fitted to the recess is provided at the other one of the fastening seat portion of the cylinder head and the flange of the exhaust manifold, and the exhaust manifold is fastened to the cylinder head by placing the flange of the exhaust manifold on the fastening seat portion of the cylinder head in a state where the protrusion is fitted to the recess and wherein the exhaust manifold mounting structure further comprises: a knock-pin provided at a center portion of the fastening seat portion of the cylinder head; and an insertion hole, to which the knock-pin is inserted, and which is provided at a center portion of the flange of the exhaust manifold, wherein the knock-pin is inserted in the insertion hole in a state where the flange of the exhaust manifold is placed on the fastening seat portion of the cylinder head. a recess is provided at one of the fastening seat portion of the cylinder head and the flange of the exhaust manifold in correspondence with an area in between any adjacent two of the exhaust port outlets (4a, 4b, 4c, 4d), and a protrusion that is fitted to the recess is provided at the other one of the fastening seat portion of the cylinder head and the flange of the exhaust manifold, and the exhaust manifold is fastened to the cylinder head by placing the flange of the exhaust manifold on the fastening seat portion of the cylinder head in a state where the protrusion is fitted to the recess and wherein the exhaust manifold mounting structure further comprises: a knock-pin provided at a center portion of the fastening seat portion of the cylinder head; and an insertion hole, to which the knock-pin is inserted, and which is provided at a center portion of the flange of the exhaust manifold, wherein the knock-pin is inserted in the insertion hole in a state where the flange of the exhaust manifold is placed on the fastening seat portion of the cylinder head. a knock-pin provided at a center portion of the fastening seat portion of the cylinder head; and an insertion hole, to which the knock-pin is inserted, and which is provided at a center portion of the flange of the exhaust manifold, wherein the knock-pin is inserted in the insertion hole in a state where the flange of the exhaust manifold is placed on the fastening seat portion of the cylinder head.