PURPOSE: To prevent the abrasion of a lower shell, and the like by engaging the engaging portions of a revolving vessel with engaging brackets provided on the inner bottom face of the lower shell at a defined looseness while providing base seats between the inner bottom face of the lower shell and the outer bottom face of the revolving vessel leaving a gap.

CONSTITUTION: A revolving vessel 5 is installed on the inner bottom face of the lower shell 2 of a fuel tank 1. A number of engaging tongue pieces 7a-7d each having an engaging slit 6 extending in the horizontal direction are projectingly provided on both end portions of the revolving vessel 5. On the other hand, a number of engaging brackets 9a-9d each having an engaging hook 8 which is engaged with each engaging slit 6 at a defined looseness (h) are provided on the inner bottom face of the lower shell 2. A number of base seats 10a-10d are projectingly provided while leaving a gap H between the outer bottom face of the revolving vessel 6 and the inner bottom face of the lower shell 2, on four corner portions on the outer bottom face of the revolving vessel 5 which are opposed to each base portion 9a1-9d1 of each engaging bracket 9a-9d.