PURPOSE: To eliminate the possibility of oil rising along the inner wall of a pouring opening and leaking out by providing, breather holes at the place other than the outer peripheral part of a cap body in order for the pouring opening to be sealed by the cap body in close contact with the entire peripheral edge thereof.

CONSTITUTION: The oil flows through a gap 69 of a third baffle member 35 and reaches a gap 57 between a second baffle member 33 and the third baffle member 35. However, since the gap 57 is narrow and a cushioning chamber 55 is wide, capillary action is difficult to occur. Consequently, the oil will not leak out through breather holes 61-65. Even if the oil is allowed to enter the cushioning chamber 55, since the third baffle member 35 is provided with a sloping part 49, the oil drops off easily along the sloping part 49 and is sent back into a tank 1. The oil flowing through the gap 69 of the third baffle member 35 will not leak out therefrom, since the inner wall of a pouring opening 3 is obstructed by inner holding parts 11. Also, since the oil is sealed by the inner holding part 11 and a projecting part 47, it becomes difficult to leak out from there.