The SmA, SmCα* and SmC* phases of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl) phenyl 4‘-octyloxybiphenyl-4-carboxylate (MHPOBC) have been studied by means of simultaneous measurements of the dielectric constant and the electroclinic coefficient under a dc bias electric field. From the three dimensional representation of the electroclinic coefficient as a function of dc electric field and temperature, it is confirmed that there is no helix with a long pitch in the SmCα* phase and the electric field-temperature phase diagram for these phases is obtained. In the SmA phase near the phase transition temperature, the temperature dependences of the dielectric constant and the electroclinic coefficient are analyzed in terms of the phenomenological theory of Landau type and the parameters appearing in the theory are determined.