The complex dielectric constant of a ferroelectric liquid crystal was measured under pressures up to 350MPa. The chiral smectic C (SmC*)- smectic A (SmA) transition temperature, T-C, increases with an initial slope of dT(C)/dp=0.19 K/MPa. ne real part of the complex dielectric constant in the SmC* phase decreases as the pressure increases. The dielectric strength, Delta epsilon (G), of the Goldstone mode shows an initial decline for pressures of up to about 100MPa, and then approaches a constant value, and the relaxation frequency, f(G), of the mode decreases steadily. Taking into account of the phenomenological relations of Delta epsilon (G) proportional to - 1/K and f(G) proportional to K/gamma (K: elastic constant, gamma: rotational viscosity), we conclude that the reduction of the dielectric constant in the SmC* phase is mainly caused by a decrease in the relaxation frequency due to an increase in the rotational viscosity.