An inflatable bag,for airbag restraint systems for a front seat passenger, of a quadrilateral prismoidal shape comprising a right-angled quadrilateral base portion adapted to face and receive the passenger by the extension of the bag upon deployment, a lip portion defining an opening for admitting gas from the system and adapted to be attached to a dashboard, upper and lower face portions, and lateral face portions having T-like or Y-like joints diverging from the lip to base portions and exhibiting a strap effect. The bag is made of a one-piece cloth, each outlining lateral side of which has longitudinally consecutive six sides of three triangular sections wherein two endmost sides and respective two adjoining sides have respectively an equal length, or longitudinally consecutive five sides wherein two endmost sides have an equal length and other two sides adjacent to the former have a total length equal to the length of a longitudinal side therebetween. When the cloth is longitudinally folded into two, the endmost sides are superposed and sewed together. Then, respective adjoining sides are superposed and sewed together, thus forming the Y-like joints and lateral face portions, or other two sides are superposed onto the longitudinal side and sewed together, thus forming the T-like joints and lateral portions, thereby shaping the prismoidal body, which is turned inside out to give the bag. The endmost sides may assume an arc-like line.