A polyester which is characterized in that(1) it comprises ethylene terephthalate units as the main constituting component,(2) its intrinsic viscosity is from 0.6 to 0.9 dl/g,(3) the content of an antimony element is from 0.8 to 2.0 mmol/kg,(4) the temperature-rising crystallization peak temperature (Tc.sub.1) is at least 160.degree. C., and the temperature-dropping crystallization peak temperature (Tc.sub.2) is at most 170.degree. C. or non-existent, and(5) the number (N.sub.1) of spherulites when it is heated from room temperature to 120.degree. C. and maintained for 10 minutes is at most 1.times.10.sup.-2 /.mu.m.sup.2, and the number (N.sub.2) of spherulites when it is cooled from 300.degree. C. to 200.degree. C. and maintained for 2 minutes is at most 1.times.10.sup.-3 /.mu.m.sup.2.