A lower wall (8) forming a fuel storage chamber (5) becomes curved downward according to increase of the fuel amount, and at this time a support member (23) for supporting the fuel storage chamber (5) does not restrain the downward curvature of the lower wall (8), whereby the capacity of the fuel storage chamber (5) can be increased and whereby the amount of the fuel that can be stored in the fuel storage chamber (5) can be maintained at the maximum, as compared with structure inhibiting deformation of the lower wall (8).