PURPOSE: To obtain the titled flange of opening without the decrease of pressure resistance and gas blanketing preformance in the case of sealing a metal lid by double seaming, by a method wherein the flange of the opening is formed by heating at a suitable high temperature, and pressuring the opening of container main body molecularly orientated with stretching.

CONSTITUTION: The opening of cylinder of container main body melt-extruded in a tublar state from thermal plastic polyester composed of ethylene terephthalate repeated base unit as main body and stretched, is heated at a temperature within the range of the glass transition temperature of polyester and over upto the melting temperature. The opening of the cylinder is applied with a pressure of not less than 1kg/cm2 to the direction of the wall thickness at the same time. Thus, the molecular orientation of the formed flange is relieved, the rigidity being decreased, while the number of the products defective in double seaming process is decreased, and the pressure resistance of the acceptable products is not deteriorated.