PURPOSE: To make possible to prepare a preform capable of being blow molded into a heat-resistant polyester bolt, by subjecting a pipe to heat treatment at a specified temperature for a specified period prior to the preforming thereby resulting in the crystalization.

CONSTITUTION: A polyethylene terephthalate or copolymerized polyester consisting mainly of a polyethylene terephthalate is melted, extruded and subjected to vacuum molding, and then it is quenched to obtain the transparent pipe. Prior to the preforming, one end or, if desired, both ends of the resulting are heated at a desired temperature in the range of from 90 to 200°C for from 5 to 200sec so that the one end or both the ends are crystallized. After the one end is preheated in molds to be sealed under pressure to be formed with a bottomed section, it is preheated by a preheater 1 and is worked to have a section engageable with a cap by an outer mold 4 and an inner mold 5 thereby a preform is molded. The polyester bottle obtained from the thus prepared preform is improved in heat resitance, and can be filled with seasoning or the like at high temperatures.