An Antiferroelectric liquid crystal (AFLC) CS-4001 was studied to find details of the field-induced ferroelectric phase transition by various measurements including the electro-optical hysteresis loops, switching current responses, and dielectric relaxation spectra. Contributions from two low frequency dielectric modes to the phase transition anomalies by further extended by are discussed on the basis of the phenomenological Landau theo Orihara and Ishibashi.