PURPOSE: To reliably prevent the vehicle from fire due to outflow of fuel by constituting so as to separate the cap part from the drum part in case where a filler cap hits a filler base or another.

CONSTITUTION: A filler cap 50 has a cap part 506 and a drum part 50a which are in the normal condition integrally combined by screws, and it seals a filler tube 6. At the time of vehicular collision, in case where external force as shown by arrows F1 or F2 is applied to the filler cap 50, engaging condition of the inner drum 59 of the cap part 506 with the upper end of the drum part 50a remains as it is, and rupture is given to a concave groove 53 of the drum part 50a. In this moment, since the part which is lower than the bulkhead 52 of the drum part 50a of the filler cap 50 remains screwed in the filler tube 6, there is no instance that fuel may flow out.