PURPOSE: To enhance sealing properties by a method wherein the pressure contacting faces of a formed body except the neighborhood of its swollen-out portion is cut off and, at the same time, fluid is sealed in the interior of the formed body under the condition that the boundary between the portion swollen out along an air runner and the portion swollen out along a cavity is heat-sealed and, after that, the portion swollen out along the air runner is cut off outside the sealed part.

CONSTITUTION: Blow molding is performed with molds B having annular cavities C so as to obtain sheet-like resin formed body having an annular hollow region. Next, portion excluding the hollow region is cut off. By sealing an air passage at molding under the condition that fluid is enclosed in the interior of the hollow region, an annular hollow packing is obtained. Since the fluid in the interior of the above-mentioned hollow packing flows, the packing can follow to any unevenness of sealing surface, resulting in allowing to seal the whole sealing surface by uniform pressure.