the intermediate ribs 172 may be coupled or crossed to each other so that the resultant arrangement of the intermediate ribs 172 may have a honeycomb or diagram shape in order to further support the partitioning rib 171 of the inside wall reinforcing ribs 170 a supplemental concave wall reinforcing rib 177 is provided outside of the inside wall reinforcing ribs 170 the supplemental concave wall reinforcing rib 177 comprises an auxiliary partitioning rib 174 and a plurality of auxiliary intermediate ribs 175 the auxiliary partitioning rib 174 is provided in a continuous manner along the partitioning rib 171 while being spaced from the partitioning rib 171 the auxiliary intermediate ribs 175 intersect the partitioning rib 171 and partitioning rib 174 at predetermined intervals and form a plurality of cells or compartments 176 between the partitioning rib 171 and partitioning rib 174 this construction attains further rigidity of the inner surface 161 of the space 9 in another embodiment supplemental concave wall reinforcing ribs may be provided outside of the inside wall reinforcing rib 70 of the main frame 1 a if the main frame 1 a has sufficient spare space as shown in fig1 and 12 outside wall reinforcing ribs 180 are formed in a matrix shape over nearly the entire inside of the front panel wall 252 the outside wall reinforcing rib 180 projects.