To accelerate the circulation of the oil by improving the oil capturing efficiency, and elongage the life of the oil by uniformly mixing the oil by mounting gutter along the boundary of an oil pan on an inner wall surface of a crank case, and connecting a lead pipe to a bottom of the gutter to form a capturing unit.

An inwardly projecting gutter 7 is formed on an inner wall surface of a crank case 3a of a cylinder block 3 forming an engine body 1, at a boundary of the cylinder block 3 and an oil pan 4. An oil feed pipe 8 connected to a bottom surface of the gutter 7 is led from a side part of the oil pan 4 to form a capturing unit 9 with the gutter 7. The oil flowing and dropping along the inner wall surface of the crank case 3a is completely captured in the gutter 7, and guided into a sub-oil tank 10 through the oil feed pipe 8. On the other hand, the residual oil in the sub-oil tank 10 is recycled to an oil reservoir 2 in the oil pan 4 through a circulation pipe 11. Whereby the circulation of the oil is accelerated and the oil can be uniformly mixed.