To simplify the construction of an air membrane as well as the maintenance of strength and manufacture thereof by using a membrane material of bias-woven fiber textile coated with resin material, etc., and connecting both sides of a plurality of belt-shaped membrane pieces along the plane diametral direction to form them in the shape of a dome.

Both side edges of each of membrane pieces 2 constituting an air membrane 1 are so formed that they have parallel chords 3 and 3 along the same direction, the parallel chords 3 on both sides of a plurality of membrane pieces 2 are overlapped each other and are sewed at joint sections 4, and they are jointed by means of heat welding to form. The air membrane 1 is not closely connected because a connection interval between the joint sections 4 and 4 is broad, and local stress is not concentrated in the joint sections because the whole air membrane is a two ply membrane. The air membrane 1 is formed of the membrane pieces 2 having broad area with average strength. The membrane pieces 2 are adopted from bias-woven fiber textile to reduce loss in the case it is cut out, at the same time, vertical and horizontal extension can be the same, the extent of expansion in the direction along the parallel chords 3 and the direction at right angles can be the same in the case air pressure is provided, and a continued average dome can be formed.