To further accurately control a quantity of scavenging air, in a longitudinal scavenging-type crosshead large-sized engine.SOLUTION: An engine comprises cylinders 10 having cylinder liners 5, and cylinder sections 4 having bottom parts 3. A plurality of the cylinder liners are arranged while being aligned in the cylinder sections in a longitudinal direction A. Pistons are arranged in the cylinder liners 5, respectively. A scavenging air opening part 7 is formed while adjoining end parts 51 of the cylinder liners 5, and the scavenging air from a scavenging air receiver 8 is thereby made to flow therein. Piston sub-spaces 11 which are defined by lower faces of the pistons of the cylinders 10 at one side, and defined by the bottom parts 3 of the cylinder sections 4 at the other side are formed for the cylinders 10. A separation wall 12 is arranged so as to separate the adjoining piston sub-spaces 11 from each other. The piston sub-spaces 11 are connected to the scavenging air receiver 8 via a flow connection part 13.SELECTED DRAWING: Figure 1