To provide a fuel tank support structure that is easily protected from heat and copes with thermal deformation of a tank even if a tank housing part includes a multistage part, and to provide a floating structure.

The fuel tank support structure is provided to mount a fuel tank 2 for housing fuel to be used for propelling a floating structure 1 or driving an accessory device, on a housing part 3 formed in the floating structure 1. The fuel tank support structure includes a step-like multistage part 31 formed on the bottom part of the housing part 3, a plurality of support blocks 4 arranged in the multistage part 31, and a tank bottom part 21 formed in the fuel tank 2 along the multistage part 31. The fuel tank 2 is configured to be thermally deformed depending on the capacity of the fuel stored and to have larger number of support blocks 4 for supporting the tank bottom part 21 when the fuel tank 2 is filled with fuel than that when the fuel tank 2 is empty.