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 disposing a projecting portion at a crystallized upper surface of flange part, the projecting portion at at least heat-sealing-face constituting region rendered amorphous or of low crystallinity. Further, in order to suppress variation of strength of sealing with a cover member and stably attain high sealability, the projecting portion is deformed at the time ofheat sealing with the cover member so as to form a resin piece protruding toward the interior of the container while being substantially in close contact with the upper surface of opening rim part. The formed resin piece couples with a sealant layer of the cover member.