PURPOSE: To automatically open a booklet by utilizing the expanding force of a fastened expansible bag.

CONSTITUTION: An expansible bag is held between the leaves 2, 2 of a booklet 1 and expanded by the following methods not only to open the booklet 1 but also to allow a three-dimensional article to appear. The bag is expanded by a method wherein sodium bicarbonate and an acidic soln. are received in the expansible bag 13 in a separately partitioned state and the partition is pierced by operating a sharp jig such as a needle or weakly formed to be broken by external force to expand the bag by generated carbon dioxide or a method wherein sodium bicarbonate and tartaric acid are charged in the expansible bag 13 and water is injected into the bag from an injection port to expand the bag by generated carbon dioxide or a method wherein a blowing-in port is provided to the expansible bag 13 and air is blown in the bag to expand the bag or a method wherein a platen is folded double to form a two-leaf booklet 1 and the expansible bag 13 is fastened between leaves 2 and expanded by the above means to automatically open the booklet.