To instantaneously fabricate a flat base sheet into a form for use, and to put the fabricated form instantaneously back to the state of flatness on occasion of nonuse.

An auxiliary flap 11, capable of being turned up along a crease 12 toward the reverse side, is provided at least at one side of a flexible base 10 made of paper or the like, and strips 13, each flanked from both sides with a pair of incisions provided at the surface of the base 10 in the direction intersecting the side of the base at a right angle, are provided. Each of the strips 13 is formed with its end 15A on one side positioned, crossing the crease 12, at the inside of the auxiliary flap 11. With the auxiliary flap 11 turned up toward the reverse side of the base 10 on occasion of use, each of the strips 13 can be raised to form an archlike shape.