In a cylinder block for an internal-combustion engine, a cap-mounting bulk portion is connected with both first and second side walls of a crankcase formed below a lower deck. Each of first and second bulk connection portions connecting the first and second side walls, respectively, with the cap-mounting bulk portion projects inwardly from the corresponding side wall. The bulk connection portion is formed with a hollow portion extending in a vertical direction of the cylinder block. The first and second bulk connection portions are connected, respectively, with first and second head bolt bosses, which are formed on an upper block wall surrounding an upper part of a cylinder of the engine, by first and second connecting ribs extending in the vertical direction.