To achieve high sensitivity, high-speed response, and wide range, a force sensor with quartz crystal resonator, which has the characteristic that the resonance frequency changes by the external force, is fabricated with a simple structure. AT-cut quartz crystal is employed due to its superior temperature and frequency stability. Because it is weak for the quartz crystal to the impact, bend, and tension, an element retention mechanism was designed that safely maintains the quartz crystal resonator for the external force with flat structure. In this study, a novel retention mechanism of the quartz crystal resonator for the flat structure was fabricated and analyzed. The proposed force sensor is flat, small, and sensitive to be applied to several usages such as medical treatment and dynamic contact force detection of human.