To approximately completely discharge the polluted lubricating oil by mounting an air inlet on a cap of a lubricating oil inlet of a combustion engine, naturally making the polluted lubricating oil flow down to be discharged from a drain port in the replacement of the lubricating oil, and blowing the compressed air from the air inlet to forcibly discharge the residual lubricating oil.

A lubricating oil filling cap A mounted on an air inlet B is mounted on, for example, a lubricating oil filling port of an automobile engine. This air inlet B is same as, for example, an air inlet of a tire, and any air inlet will do so far as it is closed by a cap even through it does not have a valve. To replace the lubricating oil of the automobile engine, the lubricating oil is naturally made to flow down to be discharged from a drain port of an oil pan, and then the compressed air is blown from the air inlet B to forcibly discharge the lubricating oil attached and remained in the automobile engine. Whereby the polluted lubricating oil can be approximately completely discharged, and the lubricating oil to be newly filled, can be prevented from being polluted.