In a first step, a sample liquid is fabricated by mixing a sample formed of a polyester or decomposition products of a polyester into a solvent. The solvent used here contains chloroform and 2,2,2-trifluoroethanol and has an organic base added thereto. The organic base is at least one of a primary amine, a secondary amine, a tertiary amine and a heterocyclic amine. Next, in a second step, each of the amount of the terephthalic acid terminal and the amount of the hydroxy group terminal in the sample liquid is measured through a nuclear magnetic resonance spectroscopy aiming at hydrogen atoms. Subsequently, in a third step, the number average molecular weight of the sample is calculated from the amount of the terephthalic acid terminal and the amount of the hydroxy group terminal measured in the second step.