To elucidate the mechanism of the cyclic oligomer formation during the melt of poly(ethylene terephthalate)(PET), a labeled repeating unit composed of mone ($eta$-hydroxy-ethyl) terephthalic-d4 acid was introduced to PET chain end. It was identified that part of the content of the cyclic oligomers formed during the melt of labeled PET contained the labeled repeating unit, and was concluded that cyclodepolymerization by the action of hydroxyl end groups to the ester linkages of PET contributed to the formation of the cyclic oligomers. In confirmation of this conclusion, hydroxyl end groups of PET were blocked with benzoyl chloride. PET containing reduced cyclic oligomers was obtained as a result of the blocking.