PURPOSE: To simplify maintenance by elongating oil exchange period in an engine of sub-tank supplementary feed type.

CONSTITUTION: A sub-tank 1 is arranged on an engine E, and communicated with an oil pan 4 of the engine E through an oil supply passage 3 by means of an oil supplementary feed pump 2. The oil supplementary feed pump 2 is driven based on detection of a level switch 17 which detects an oil level in the oil pan 4, for supplying the oil in the sub-tank 1 to the oil pan 4. A second sub-tank 18 is communicated with the oil pan 4 through a communication pipe 21, while an upper space 22 in the second sub-tank 18 is communicated with a crank case 5 of the engine E through a communication pipe 23. A part of oil housed inside the oil pan 4 is supplied to the second sub-tank 18 at the driving time of the engine E.