Abstract: A fuel tank supporting structure for supporting a fuel tank (7) under the floor of a vehicle (1) is comprises a pair of right and left tank belts (8) each stretched in the vehicle longitudinal direction at two sites separated from each other in the vehicle width direction to support the fuel tank. Right and left fixing points (81) on the vehicle front side of the tank belts are each disposed aside in the vehicle width direction with respect to right and left fixing points (82) on the vehicle rear side so that each of the tank belts extends slantwise with respect to the vehicle longitudinal direction in plan view. In the bottom surface portion of the fuel tank supported by the tank belts, right and left engagement grooves (71, 72) engaging with the tank belts are provided.