We have been developing a new mobility-aid robotic vehicle, Tread-Walk 2 (TW-2), which supports walking for the elderly. TW-2 is controlled by the natural walking movement, especially using ground reaction force during gait. In our previous work, we tried to estimate the user's anteroposterior force from the motor current value without a force sensor. But, a user of this vehicle experienced some discomfort both when he started walking and when he stopped walking. This problem is caused by inaccurate estimation of the user's anteroposterior force at the heel contact and the toe off. The estimation of the user's anteroposterior force is greatly related to inaccurate estimation of the vertical component of the ground reaction force, which is approximated by the square waveform in the stance phase. In this paper, we proposed the new approximation that reflects the characteristics of the ground reaction force during the human gait. This paper describes the novel method to approximate the waveforms of the vertical forces as isosceles trapezoidal waves. By comparing the estimated anteroposterior force using the new method with the measured value using the force plate, for two young subjects whose physical characteristics were different, the waveform pattern of the estimated force was found to be similar to that of the measured force. This showed that the proposed method could possibly be useful for accurate estimation of anteroposterior force. The technology to estimate the user's anteroposterior force accurately is a key to construct a control algorithm to improve the operability of TW-2.