To make even a thin book storable and holdable by expanding the movable range of a partition plate to side boards, and to prevent cracks in the partition plate.

An upper back board 5 and a lower back board 6 are put between the both side boards while keeping a space 8 between the boards. Guiding grooves 9, 10 are formed on the upper and lower back boards 5, 6. A cut part 12 is made on a partition plate 11, and a groove for fitting 13 is formed along an inner periphery of the cut part. A side of a fitting part 14a on the partition plate side a connecting metal 14 is fitted to the groove for fitting 13, and fitting parts 14b, 14c on the back board side are fitted to the guiding groove 13 of the both back board 5, 6, slidably.