Fuel tank arrangement structure for vehicle

Disclosed is a fuel tank arrangement structure for a vehicle, which is designed to position a fuel tank 30 under a floor panel 4 forming a floor of a passenger compartment of the vehicle. In the fuel tank arrangement structure, at least a laterally inward portion of the floor panel 4 is formed as a convex floor portion 7 protruding upward from a general surface 8 of the floor panel 4, and at least a laterally inward portion of the fuel tank 30 is formed as a convex fuel-tank portion 30a protruding upward. The convex fuel-tank portion 30a is positioned within a space defined by the convex floor portion 7 while positioning the remaining portion of the fuel tank 30 at a height position lower than the general surface 8 of the floor panel 4, and a seat 9 (10) is positioned inside the passenger compartment and above the convex floor portion 7.