The whole open-structured floor such as the center tunnel and the lower part of fuel tank is thoroughly reinforced by the trussed frame that combines the tenacious steel pipe and gusset. It reduces the vibration and the kink of the floor to allow proper function of the front and the rear suspensions. When used in combination with the tower brace set, the upper/down two-stage high-rigidity body is completed. Front and rear make a set.