We have been developing a new vehicle, Tread-Walk 2 (TW-2), which supports walking for the elderly. TW-2 is controlled by the natural walking movement. In our previous work, we tried to estimate the user's anteroposterior force from the motor current value without a force sensor in order to possible to have a more intuitive operability of TW-2. 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 order to construct a control algorithm to improve the operability of TW-2, we accurately estimated the user's anteroposterior force. We therefore proposed the new method to approximate the vertical force in order to estimate the user's anteroposterior force accurately. This paper describes the new method to approximate the waveforms of the vertical forces as isosceles trapezoidal waves with a slope changed depending on the change of the walking velocity. 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.