In the injection molding of polymer lenses, the demolding resistance may cause problems, such as deformation, cracks, and scratches, in these products. To examine the method of decreasing the demolding resistance of polymer lenses, a method of precisely measuring the demolding resistance should be established. In this study, we designed and fabricated a mold equipped with high-sensitivity quartz force sensors for demolding resistance measurement that is customized for the injection molding of polymer lenses. In addition, we evaluated the performance of the mold. Using this mold, we measured the demolding resistance of fluorine-based polyester lenses. As a result, it was clarified that (1) the preload applied to the quartz force sensors before the start of mold closing has a significant effect on the generation of demolding resistance, (2) the draft angle of a cavity wall has an effect on the demolding resistance, and (3) the maximum demolding resistance increases with increasing holding pressure, injection rate, heating cylinder temperature, and mold temperature.