By applying the HAUP (high‐accuracy universal polarimeter) method, [N(CH3)4]2CuCl4was found to be optically active in its incommensurate phase, e.g.,g23=3.9×10−5at 24 °C. The temperature dependence ofg23was obtained. By making use of the fact that the commensurate phase of [N(CH3)4]2CuCl4is ferroelastic and centrosymmetric, the temperature dependence of the soliton density in its incommensurate phase was deduced by this optical measurement. It is suggested that the HAUP method will become a new and direct method for measuring soliton densities of incommensurate states.