an air - tight filler neck cap wherein an operating torque is amplified to obtain a large tightening torque including an outer cap of the filler neck which includes a knob and a cover grip which is engaged with a body of an automobile to thereby prevent the cover grip from rotating and an inner cap of the filler neck cap which is moved in association with the outer cap by a planetary gear mechanism . the planetary gear mechanism includes a sun gear , planet gears and a ring gear located on a carrier disk plate which is rotatably attached to an upper face of the inner cap .