This study aims to develop an effective measurement instrument and analysis method of gait stability, particularly focused on the motion of lower spine and pelvis during gait. Silicon micromechanical inertial instruments have been developed and body-attitude (pitch and roll) angles were estimated via closed-loop strapdown estimation filters, which results in improved accuracy of estimated attitude. Also, it is shown that the spectral analysis utilizing the Fast Fourier Transform (FFT) provides an efficient analysis method, which provides quantitative diagnoses for the gait stability. The results of experiments on various subjects suggest that the proposed system provides a simplified but an efficient tool for the evaluation of both gait stabilities and rehabilitation treatments effects.