Abstract: A polyester container furnished with a flange part having mechanical strength, transparency, and heat resistance through orientational crystallization, which polyester container exhibits low-temperature heat sealability, is provided by forming a projecting portion at an upper surface of the crystallized flange part, the projecting portion at at least a heat-sealing-face constituting region rendered amorphous or of low crystallinity. Further, in order to suppress the unevenness of the strength of sealing with a cover member and stably attain the high sealability, the projecting portion is deformed at the time of heat sealing with the cover member so as to form a resin piece protruding toward the interior of the container while being substantially appressed against the upper surface of an opening rim part. The formed resin piece couples with a sealant layer of the cover member.