PURPOSE: To eliminate an unsatisfactory operation by providing two or more openings communicated into a cavity of a float and providing a blocking means at least on one opening.

CONSTITUTION: The other end of the air vent fine tube 9 of a feed oil pump is connected to the discharge opening section of a float 12 rotatably provided by a rotary shaft 11 at the opening end of a discharge hose 10. The float 12 has a cavity 14, a ball 15 is at rest in the center recessed hole 18 of a dish- shaped mount bed 17 on the intake opening section 16 side provided on the nearly opposite side to a discharge opening section 13 to block the opening section 16. When the float 12 is inclined, the ball 15 is rolled, the intake opening section 16 is opened, and air enters into the cavity through an air guiding pipe 23. Accordingly, an unsatisfactory operation can be eliminated with a very simple structure.