PURPOSE: To improve spreadability of a bag and prevent rupture thereof by equalizing the thicknesses of the parts of the bag folded in many narrow layers and winding the bag evenly around an inflated part.

CONSTITUTION: A bag which is mounted on an inflated part 3 is symmetrically folded in the first and second quadrants S1 and S2 with respect to a line Y-Y and then folded in many narrow layers from the crossing point of both the folding lines toward the inflated part which is centered on the plane of the bag. The bag is then symmetrically folded in both the sets of the first and third quadrants S1 and S3 and second and fourth ones S2 and S4 with respect to a line Y-Y as to close the inflated part 3, thereby defining a substantially semi- circular shape. The arc part of the semi-circular part is folded toward the inflated part 3 in many narrow layers and both ends of the folded part are wound around the inflated part and fastened with appropriate fixing means.