A supporting structure for a fuel tank in a vehicle includes a suspension arm fixing structure provided under the floor of a vehicle body and having a statically sufficient rigidity, the fuel tank being located so as to partially overlap the suspension arm fixing structure in the longitudinal direction of the vehicle, and a supporting member provided under the fuel tank so as to extend in the lateral direction of the vehicle being detachably fastened to the suspension arm fixing structure to bring the fuel tank into pressure contact with the floor of the vehicle body by fixing the fuel tank. The supporting member serves also as a reinforcing member for imparting rigidity to the suspension arm fixing structure so that the suspension arm fixing structure can sufficiently sustain a dynamic load during running of the vehicle.