Abstract: A cylinder block comprises bearing sections for supporting a rotatable shaft disposed within a crankcase inner chamber, the bearing sections being spaced from each other and from main bearing sections for supporting a crankshaft; and generally cylindrical hollow beam members each of which is interposed between the two opposite bearing sections in a manner to cover the rotatable shaft supported by the bearing sections, the hollow beam members being aligned in the direction of the row of cylinder barrels, thereby effectively increasing the total rigidity of the cylinder block while achieving engine weight-lightening.