To reduce the number of parts, improve the mountability and maintainability, and allow for a reduction in the size and weight an engine is provided having a crankshaft, a crankcase having a plurality of bearing portions for rotatably supporting the crankshaft with each of the bearing portions being dividable along a divisional plane arranged on the axis of the crankshaft. Cylinder barrels are connected to the crankcase and include cylinder bores with cylinder heads connected to the cylinder barrels. The cylinder heads are integrated with portions of the cylinder barrels forming at least the cylinder bores to form cylinder blocks. The cylinder blocks and the crankcase are fastened together by a plurality of fastening bolts having axes parallel to the axes of the cylinder bores and extending through at least the cylinder blocks, with a compressive structure in the axial direction of the fastening bolts.