To provide a folding jig for surely transporting a plate material in a folded state while preventing the plate material from slipping off, and simplifying the assembling work of the plate material of large size, and to provide a foldable storage using the folding jig.

This folding jig 5 is fixed in a state of pressing one end part 52 having a slip-off preventing structure 521, into a hole formed on an end grain face of the plate material 6a. The other end part 53 is inserted into a hole 8 formed on an end grain face of another plate material 6b, and fixed by a fixing member 9. When the storage is assembled from this state, the fixing member 9 is loosened to be removed from a fixing part 531, and then the fixing part 531 which becomes movable is slightly drawn out from the plate material 6b. The other plate material 6a is rotated by 90° in this state, and then the fixing member 531 is inserted into the plate material 6b again, and fixed by the fixing member 9.