PURPOSE: To improve rigidity while ensuring the opening area of an oil return hole by providing the oil return hole near both side walls of an oil pan upper block and an oil guide and reinforcement part longitudinally forming a grooved recessed part.

CONSTITUTION: The cylinder block 1 of an engine has a skirt part 2 on the lower part, and a lower block (oil pan upper block) 5 having four-side circumferential wall parts 2 and a bottom plate part 4 is connected to its lower end. The lower block 5 has four bulk heads lined in the direction of a crank shaft. In this case, oil return holes 13, 14 are provided near the circumferential wall part 3 on both side parts of the lower block 5. The plate part 4 edge laid along the oil return hole 13 on the rotating directional upper stream side of a connecting rod is formed with a sharp angle, and a vertical wall rib 15 serving as an oil guide is formed on its lower part. The oil return hole 14 is formed between this edge of the plate part 4 and a ridge rib 16 protruding from the circumferential wall part 3 side on a different level above this edge.