A method of preventing misassembly of an internal combustion engine, used when crankcases (2, 102) and cylinder blocks (3, 103) of first and second similar internal combustion engines (1, 101), are assembled together with each other with their mutually facing mating surfaces (2f,3f; 102f,103f) in contact with each other, by passing stud bolts through bolt insertion holes in insertion hole boss portions (3B; 103B) formed around the cylinder blocks, so as to be screwed into threaded holes (2h; 102h) in threaded hole boss portions (2B; 102B) formed around the mating surfaces (2f; 102f) of the crankcases (2, 102), characterized in that: among the crankcase and the cylinder block of the first internal combustion engine, mating with each other with their mating surfaces (2f, 3f) in contact with each other, the crankcase of the first engine is formed with a side wall facing the cylinder block of the first engine, the side wall being formed with an abutting interference portion projecting, toward the cylinder block of the first engine, from one of the insertion hole boss portions at the mating surface of the crankcases of the first engine, the side wall being further formed with a reinforcement rib extending from the abutting interference portion, the cylinder block of the first engine being formed, at a position opposing the abutting interference portion, with no interference portion to be abutted against by the abutting interference portion when assembling; among the crankcase and the cylinder block of the second internal combustion engine, mating with each other with their mating surfaces (102f, 103f) in contact with each other, the crankcase of the second engine, is formed with a side wall facing the cylinder block of the second engine, the side wall being formed with no projecting abutting interference portion; the cylinder block of the second internal combustion engine, in case the cylinder block is an erroneous one, is formed with an abutted interference portion, at a position confronting the abutting interference portion of the crankcase of the first engine, to be abutted by the abutting interference portion, the abutted interference portion projecting from one of the insertion hole boss portion formed around the cylinder block of the second engine; and when the cylinder block of the second engine is assembled with the crankcase of the first engine, misassembly is prevented due to interference-abutting engagement of the insertion hole boss portion of the cylinder block of the second engine with the abutting interference portion of the first engine. among the crankcase and the cylinder block of the first internal combustion engine, mating with each other with their mating surfaces (2f, 3f) in contact with each other, the crankcase of the first engine is formed with a side wall facing the cylinder block of the first engine, the side wall being formed with an abutting interference portion projecting, toward the cylinder block of the first engine, from one of the insertion hole boss portions at the mating surface of the crankcases of the first engine, the side wall being further formed with a reinforcement rib extending from the abutting interference portion, the cylinder block of the first engine being formed, at a position opposing the abutting interference portion, with no interference portion to be abutted against by the abutting interference portion when assembling; among the crankcase and the cylinder block of the second internal combustion engine, mating with each other with their mating surfaces (102f, 103f) in contact with each other, the crankcase of the second engine, is formed with a side wall facing the cylinder block of the second engine, the side wall being formed with no projecting abutting interference portion; the cylinder block of the second internal combustion engine, in case the cylinder block is an erroneous one, is formed with an abutted interference portion, at a position confronting the abutting interference portion of the crankcase of the first engine, to be abutted by the abutting interference portion, the abutted interference portion projecting from one of the insertion hole boss portion formed around the cylinder block of the second engine; and when the cylinder block of the second engine is assembled with the crankcase of the first engine, misassembly is prevented due to interference-abutting engagement of the insertion hole boss portion of the cylinder block of the second engine with the abutting interference portion of the first engine.