There is a growing need for a gait analysis system that can be used for personal use at low cost and for a long time. Also, even in sports, there is a great need for a system that can be used by the player at all times during practice of daily running and that can perform motion analysis without a force plate. Accordingly, we aim to develop a sensing system to estimate the floor reaction force from the acceleration of the body without using a force plate. Firstly, we measured the accelerations of 15 parts of the body during walking with a motion capture system, and confirmed whether the floor reaction force can be estimated from inertial and gravity force. Next, we tried to decrease the number of sensing point by camera and examined the accuracy of the estimated ground reaction force, and confirmed there is no big difference between the acceleration measured by the camera and by the inertial sensor. Finally, frequency response analysis for the measured acceleration during walking was carried out and the relationship between the motion of the trunk and other parts were clarified.