To facilitate replacement of a core rib by forming a core rib bag body to/from which the core frame can be inserted/removed.

A core rib 2 is made of a PRP pole and the like provided with high flexibility and highly deformability. A cage is formed by binding the end parts of a core rib bag body 3 together by machine sewing. As the connecting parts in the cage need flexibility for providing the foldable cage, the length of the core rib 2 is shortened as against that of the core rib bag body 3. The cage is formed by arranging the core rib bag body 3 without inserting any core rib 2 into any side face ridge part 12 and sewing and fixing a mesh body 1 to the core rib bag body 3. When the core ribs 2 are inserted into upper face opening ridge parts 11 and into the side face ridge parts 12, the respective ridge parts are stretched and stand by themselves so as to form a box shape. On the other hand, in this cage, respective corner parts 8 are flexibly connected to each other, so that the cage can be flattened when the respective ridge parts are folded up even if the core ribs 2 are inserted, so that it is convenient for storage.