the rear window opening accordingly the mounting work can be readily performed in fig7 the front portion 25a of the rear floor 25 to be positioned under the rear seat 6 is a place for housing a spare tire right and left recesses 25d 25d in the middle portion 25b correspond to rear wheel houses the space between the recesses 25d 25d is the place for arranging the fuel cylinders 3 4 and the rear portion 25c constitutes a bottom wall of the trunk room 8 the rear floor 25 is supported on a pair of right and left rear side frames 24 24 directing in the before and behind direction the right and left side supporting frames 10 10 of the supporting frame 5 are positioned on side edge portions along the right and left recesses 25d 25d of the middle portions 25b with the front and rear attaching parts 10a 10b of the side supporting frames 10 put on front and rear attaching holes 25e 25e formed on the side edge portions the supporting frame 5 with the fuel cylinders 3 4 attached is tightened to the rear floor 25 together with the lower rear side frames 24 by bolts to be fixed to the car body next connection of ends of pipes provided in the neighborhood of the fuel cylinders 3 4 is carried out as shown in fig7 there is a opening.