We prototyped a new device with a soft and flexible pressure sensor to measure the force to close the mandible with or without occlusal support (jaw-closing force, JCF). This study aimed to clarify the practicality of this instrument. Healthy young and older adults with occlusal support were recruited. Intra- and inter-rater reliability of the JCF meter was examined using data from younger participants. Data regarding age, sex, body mass index, remaining teeth, and dentures of the older adults were obtained. Furthermore, the right and left JCFs were measured using a JCF meter; occlusal force was measured using an existing occlusal force-measuring device. Intra- and inter-rater correlation coefficients were significantly reproducible (0.691–0.811, p < 0.05). JCF was correlated with occlusal force (p < 0.05). Multiple regression analysis revealed that factors significantly associated with JCF included denture status (p < 0.001), age (p = 0.038), and occlusal force (p = 0.043). The prototyped JCF meter can measure JCF with high reproducibility, reliability, and validity. Further, association with occlusal force, which is an existing index, was observed. This device could be used to measure the JCF with or without occlusal support as a new method of evaluating oral function in older adults.