Dielectric dispersion has been studied in the frequency range of 10Hz divided by 1MHz in a liquid crystal with 2-alkoxypropionate chiral group, which exhibits the helix twist inversion in the SmC* phase at about 109 degrees C. The dispersion reveals two modes, dielectric strengths as well as relaxation frequencies of which exhibit a strong dependence on the sample thickness. Both modes are also detected in the electrooptic response. The mode with the relaxation in the kHz range is the Goldstone mode, which is combined with the thickness mode. In the temperature range near the inversion temperature, where the helicoidal structure is unwound, it becomes the pure thickness mode. The origin of the lower frequency mode, which is detected for the first time, remains unexplained.