The tank mounting structure includes a plurality of tanks, a frame on which the plurality of tanks are mounted in a state of being aligned in the width direction, and a plurality of mounting structures that are fixed to both end portions of the frame in the axial direction and support each tank. The tank has a first neck arranged on one side in the axial direction and a second neck arranged on the other side. The first neck is fixed to the frame via the first mounting structure. The second neck is supported by the frame via a second mounting structure and is movable in the axial direction by expansion or contraction of the tank. The first necks of adjacent tanks are alternately fixed to the end portions of the frame in the axial direction.