Stabilometry, which is useful for the quantitative evaluation of the equilibrium function, is typically performed using Romberg's posture. In this posture, stabilometry can detect deterioration in the equilibrium function because this posture is unstable with a small support area. For many elderly individuals and for patients with equilibrium disorders, it may be difficult to maintain Romberg's posture for several seconds without falling to the floor during stabilometric analysis. The purpose of this study was to investigate the control system under unstable conditions. We developed an apparatus for adjusting the tilt angle of the slope on which body sway is recorded. Sixty-nine healthy paid volunteers (13-87 years old) participated in this study. Their body sway in Romberg's posture was recorded, and the temporally averaged potential function was estimated from the stabilograms. We then compared the mathematical models for body sway on slopes with tilt angles 0 degrees, 10 degrees, and 20 degrees.