Abstract: A lubricating oil passage structure for a cylinder block having a plurality of cylinders separated from each other by journal walls and arranged in a row, bearing bores formed in the journal walls for rotatably supporting a pair of rotary shafts in parallel with the crankshaft of the engine on either side of the cylinder row, comprising: a hollow gallery member internally defining three galleries extending in parallel with the crankshaft in the hollow gallery member, and mechanically connecting the bearing caps with each other. Lubricating oil passages extend from these oil galleries to the bearings for the crankshaft and the rotary shaft linearly and in parallel with the bolts for securing bearing caps for the crankshaft bearings to the journal walls. Thus, the lubricating oil passages can be formed in a single manner, and the lengths of the passages are minimized. Furthermore, the gallery member can effectively reinforce the journal walls.