An engine fastening structure in which a crankcase and a cylinder body are provided as separate units, so that the crankcase and the cylinder body are connected together with connecting bolts, characterized in that the crankcase 2 is constructed in such a manner that an iron alloy bearing member 12b for supporting a crankshaft bearing is insert cast in an aluminum alloy, and in that connecting boss portions 12c are integrally formed on both sides of the iron alloy bearing member 12b which are situated opposite across a cylinder axis as viewed in a direction in which the crankshaft extends in such a manner as to extend toward a cylinder body side, so that the connecting bolts 30a are screwed into the connecting boss portions 12c, respectively.