To easily and safely perform the opening/closing operation of a bottom valve by a method wherein the bottom valve is constituted in such a manner that a valve body is separatably arranged in a valve seat which is provided on a valve main body, and in the meantime, an air cylinder is provided on the ceiling wall of a fuel storage tank, and by this air cylinder, the valve body is open/close-operated by an interlocking mechanism.

For a tank lorry, a bottom valve 13 is attached on the bottom part 10b of a fuel storage chamber 12, and the leading end of a supply pipe of which the base end is connected to a fuel oil supply source, is connected. On the ceiling wall 10a of each fuel storage chamber 12, a fuel oil vapor inlet/ outlet 50 is provided, and at the upper end of the main body 51, a double acting type air cylinder 30 is attached. This air cylinder 30 is constituted in such a manner that by switch-operating an air passage switching valve, air in an air tank is force-fed into the cylinder main body 31, and a piston 32 is slidably raised, and by doing so, a valve body 25 is raised while resisting a pressing spring 26, by a piston rod 33, an interlocking rod 56 and a valve body supporting rod 24, i.e., the valve body 25 is opened.