Studies on footwear-based wearable devices for gait and motion monitoring have been conducted by many researchers, and the force measurement of a foot is one of the most important topics among them. In this paper, we present the design of a soft force-sensing pad for in-shoe force measurement. A force-sensing method based on an air pressure sensor is utilized to robustly measure the force caused by compression in a shoe. To withstand the human weight, an air-filled structure with an elastomeric pillar array is designed. The force is measured by the increase in air pressure caused by pillar compression. Due to the simple sensing structure, it has the advantage that it can be made into various forms such as an insole pad. To verify the sensing performance, experiments are performed to compare its accuracy, hysteresis, and repeatability with an offthe- shelf load cell. The measured results show an accuracy error of 0.249 kgf, a hysteresis error of 1.071 kgf, and a repeatability error of 0.202 kgf. To verify the robustness of the proposed sensing pad, accuracy and precision change under nine different compression conditions is calculated, and the results show an accuracy error of 0.672 kgf. Moreover, error variation of the proposed sensing pad is about four times smaller than the widely used force-sensing resistor sensor.