We developed a wearable sensor system for estimating finger contact force by measuring the mechanical deformation of the side of the fingerpad. The first part of this paper describes and validates the principle of contact force estimation, and discusses the design criteria of the wearable sensor. We then describe the manufacture of a practical sensor system in the form of a wearable fingerpad fixture and the development of a calibration procedure which eliminates the effect of individual fingerpad differences. The measured root mean square (RMS) error in estimating contact force is about 0.2 N, which is suitable for examining subtle human behaviors, such as applying cosmetics. We demonstrated the possible usage of the contact force sensor in combination with a three-axis acceleration sensor. The proposed sensor system is sufficiently small and light when fixed on the fingernail, and is considered to have little constraining or modifying effect on usual haptic behaviors. It should be widely useful for evaluating human haptic procedures in psychological research or ergonomic studies.