Abstract: An inflatable bag comprises a small sack which is charged with sodium bicarbonate and accommodating a small externally breakable container filled with an acidic solution. The small sack is enveloped in an inflatable bag member which is made of a synthetic resin having no permeability to the gas which is generated as a reaction between the sodium bicarbonate and the acidic solution. As the small container is collapsed by external pressure, the acidic solution flows out of the container into reaction with the sodium bicarbonate, thereby generating carbon dioxide gas. The small sack is punctured by the gas pressure with a sound of a bang or a whistle and, thereafter, the inflatable gas member is gradually inflated. It is possible to use a plurality of small sacks made of a material which exhibits small extension, such that one small sack is contained in another sack.