A multi-axis force/moment sensor, which can measure force and moment, is useful for robotic force control. However, the smallest multi-axis force/moment sensor has excess height, which restricts postures in tasks by a robot hand. In this paper, we aim to develop the thin-type four-axis force/moment sensor that can measure the twisting moment on the finger cushion of a robot hand and improve performance of this sensor by structural optimization, including strain gauge. As a result of applying structural optimization techniques using response surface method and desirability function on the four-axis force/moment sensor by finite element analysis, we obtained optimum design variables and validated the effectiveness of the proposed techniques. Finally, we made the production model of this sensor and performed characteristics test by applying optimum design variables. We obtained sufficient sensitivities concerning each axial load component by sensitivity evaluation of experimental results and showed the effectiveness of the developed sensor.