The present invention provides a method of manufacturing high-melting-tension heat-resistant transparent or opaque sheets, boards, and molds from polyethylene terephthalate (PET). The method is characterized in that a mixture of: (1) PET polyester whose melt flow rate (MFR) is 45-130 g/10 minutes (a); (2) coupling agent master batch (f) comprising coupling agent (d) and substrate (e), wherein coupling agent (d) is a mixture of compounds containing 2 epoxy groups (b) and compounds containing 3 or more epoxy groups (c); and (3) catalyst master batch (i) comprising coupling reaction catalyst (g) and substrate (h); is melted in a reaction-extruder to give PET polyester whose MFR is 40 g/10 minutes or less. The resulting PET polyester is molded into amorphous sheets or boards, and then heated again to a predetermined temperature to give transparent heat-resistant sheets or boards; or then vacuum-, pressure-, or vacuum-pressure-formed following heat-setting to give transparent heat-resistant or white opaque highly-heat-resistant mould articles.