To provide a structure for mounting a fuel tank on a vehicle body that can prevent harmful deformation of the fuel tank that would impair the function of the fuel tank in either case where the pressure in the fuel tank changes to a positive pressure or to a negative pressure, the structure for mounting a fuel tank on a vehicle body includes: a band lower (10L) secured to the vehicle body, extending along at least a lower wall (19L) of the fuel tank (1) and holding the fuel tank (1); a lower wall connecting means (40) that connects at least one part of the lower wall (19L) of the fuel tank (1) to the band lower (10L); a holding member (50) secured to the vehicle body and holding the fuel tank (1); and an upper wall connecting means (40) that connects at least one part of the upper wall (19U) of the fuel tank (1) to the holding member.