Novel polyester and process for preparing polyester

A polyester which comprises dicarboxylic acid constituent units derived from terephthalic acid and naphthalenedicarboxylic acid and diol constituent units derived from ethylene glycol, and which has the following properties: 40 to 99.5% by weight of constituent units derived from terephthalic acid, 0.5 to 60% by weight of constituent units derived from naphthalenedicarboxylic acid and 0 to 25% by weight of constituent units derived from isophthalic acid, based on the total amount of dicarboxylic acid constituent units, 95 to 100% by weight of constituent units derived from ethylene glycol and 0 to 5% by weight of constituent units derived from a polyalkylene glycol having an alkylene oxide chain of 2 to 10 carbon atoms, based on the total amount of diol constituent units, an intrinsic viscosity of 0.5 to 1.5 dl/g, a melting point (Tm ( DEG C)), as measured by a differential scanning calorimeter, which satisfies the following formula (I): wherein mI is the proportion (% by mol) of constituent units derived from isophthalic acid to the total amount of dicarboxylic acid constituent units, and mN is the proportion (% by molÖ) of constituent units derived from naphthalenedicarboxylic acid to the total amount of dicarboxylic acid constituent units, and an amount of constituent units which have the sequence naphthalenedicarboxylic acid-ethylene glycol-terephthalic acid (LNET (% by mol)), as measured by H-NMR, which satisfies the following formula (II): wherein mN is the proportion (% by mol) of constituent units derived from naphthalenedicarboxylic acid to the total amount of dicarboxylic acid constituent units.