To inhibit the generation of the face stepped section that may be generated between mutual partition plates when a front plate is composed of a plurality of the partition plates.

(b) in Fig. represents a b-b-line cross section in (a), and indicates an engagement with the rear of the upper partition plate 60 of engaging pieces 39 extended from the lower partition plate 30. When the partition plate 30 is fixed, the partition plate 60 is not moved to the left of Fig. by the working of engaging pieces 36. When the partition plate 60 is fixed, the partition plate 30 is not moved to the right of Fig. by the working of engaging pieces 39. (c) in Fig. represents a c-c-line cross section in (a), and indicates the engagement with the rear of the partition plate 30 of the engaging pieces 66 extended from the partition plate 60. When the partition plate 30 is fixed, the partition plate 60 is not moved to the right of Fig. by the working of the engaging pieces 66.