An oil pan (20) includes: a pair of oil pan rail portions (21a, 21b), which are spaced apart from each other under an engine body (10a), and are fastened to the engine body (10a); a pair of side wall portions (22a, 22b), facing each other, which are formed integrally with the oil pan rail portions (21a, 21b); and a cylindrical portion (23), which surrounds a driving axle (19L, 19R) passed through the oil pan (20), and is integrally joined with the pair of side wall portions (22a, 22b) near the pair of oil pan rail portions (21a, 21b). At least one axial end (23a, 23b) of the cylindrical portion (23), an upper half portion (23c) of the cylindrical portion (23) positioned over the driving axle (19L, 19R) is thickened upward up to the level near the upper surface of the oil pan rail portion (21a, 21b).