Determining accurately the amount of a fluorinated ester in the preparation thereof or determining accurately the consumption of a fluorinated ester and the amount of a formed substance in the chemical transformation of the ester enables efficient preparation of the objective substance in a yield equal to or higher than an objective level. That is, a process for preparing a fluorinated ester from starting compounds by a chemical reaction, characterized in that the reaction is continued until the yield of the ester reaches a predetermined level as determined by gas chromatography with a nonpolar column.
