A first wall is installed on a bottom portion of a housing along an outer peripheral thereof orthogonally to the bottom portion. A second wall is installed opposite to the first wall at a predetermined interval. Between the first wall and the second wall, a first rib parallel with the bottom portion for connecting the first wall and second wall is installed. Further, under the first rib, second ribs orthogonal to the bottom portion for connecting the first wall, second wall, and first rib are installed at fixed intervals. Furthermore, above the first rib, third ribs orthogonal to the bottom portion having the same height as that of the first wall and second wall for connecting the first wall and second wall are arranged so as to be shifted in position from the second ribs.