To prevent a welding miss due to neglect of changeover of a moving direction in a welding operation part, by carrying out a welding operation of welding zones 17, 18 close to a case and a welding operation of welding zones 19, 20 close to a cylinder such that the welding operation part such as a laser irradiation part and welding rod is horizontally moved.

In the fuel tank for the inclined engine, when a bottom wall part 7 close to the case and a support plate 15 close to the case are horizontally held, the welding zones 17, 18 close to the case are formed to be along a horizontal direction, and the welding zones 19, 20 close to the cylinder are formed to be along a direction perpendicular to inclined directions of a bottom wall part 8 close to the cylinder and a support plate 16 close to the cylinder. Accordingly, the fuel tank for the inclined engine is formed to be along the horizontal direction.