When human body is punched by boxing glove, both body surface and boxing glove deform in a complex shape. The purpose of this study is to develop a flexible sensor that can be used in such interface. Firstly, several mechanical phenomena, which are the cause of the error signal of the sensor, are discussed. These are the influences of out-of-plane bending deformation, shear force caused by rubbing, shear force caused by the Poisson’s effect of contact material, and the lateral compressive force caused by the overhanging deformation of flexible material. Then, a prototype sensor that can eliminate the error factors of these is developed. The sensor is a distribution type impact sensor in which sixteen sensor elements are arranged in a 4 × 4 matrix. Punching experiments using a boxing glove are carried out by installing the sensor on the load cell, on the concrete wall and on the sandbag. From the experiment, it is found that the impact force can be measured with good accuracy by using the sensor. Despite the fact that the sensor has inadequate distribution number of sensor elements, the sensor structure meets mechanical requirements for the flexible impact sensor.