A force sensor with quartz crystal resonator has the characteristic that the resonance frequency changes by the external force, which achieves high sensitivity, high-speed response, and wide range measurement by a simple structure. Also it has the superior feature in the temperature and frequency stability. On the other hand, the quartz crystal resonator had been hardly applied to a dynamic force measurement because of low degree of mechanical characteristic, that is, it is weak to the impact, bend, and tension. The objective of this study is to construct the sensor mechanism that safely maintains the quartz crystal resonator for the external force with flat structure. We designed and analyzed a novel retention mechanism of the quartz crystal resonator for the flat structure. The proposed force sensor is flat, small, and sensitive enough to be applied to several usages such as medical treatment and dynamic contact force detection of human.