PURPOSE: To prevent the output characteristic of an engine from aging, by forming an oil discharge hole in the bottom section of a subchamber extended downward from the exhaust port of an engine, so that unburnt oil pooled in the subchamber is removed through the hole.

CONSTITUTION: In a two cycle engine 10 a subchamber 14 communicated with an exhaust port 12 is arranged downward from an exhaust pipe 16. When the engine 10 runs at a low speed, the exhaust port 12 and the subchamber 14 are communicated together. Further, a part A of exhaust gas enters into the subchamber 14, and therefore unburnt engine oil contained in exhaust gas is pooled in the bottom section 14a of the subchamber 14. At this time the pooled oil is led through a discharge hole 32 formed in the above-mentioned bottom section 14a and a hose 36 connected to the latter under back-pressure of the exhaust gas, and is then dripped onto a drive chain 34. With this arrangement oil pooled in the subchamber 14 may be removed to attain a desired purpose.