We apply nonlinear dielectric spectroscopy to an antiferroelectric liquid crystal MHPOBC at various smectic sub-phases. In the SmC(alpha)* phase, the third-order nonlinear dielectric spectrum epsilon(3)* is found to be composed of the contributions from ferroelectric soft mode and soft mode of the SmC(alpha)* phase. The obtained spectrum is well ascribable by the theoretical one calculated by the Landau-type of phenomenological theory. In the SmC* phase, epsilon(3)* shows a complicated from, and is different from one observed in usual ferroelectric liquid crystals. In the SmC(F11)*(SmC(gamma)*) phase, ferrielectric Goldstone mode with broad distribution of relaxation times is observed in both epsilon(1)* and epsilon(3)*.