Horizontal subgrade reaction of a pile during soil liquefaction is investigated based on pile top vibration tests using a large-scale shear box. The subgrade reaction was resolved into elastic force and viscous force by back analysis using the Voigt model. It is shown that (1) the elastic force and the viscous force are independent of frequency under conditions which the displacement amplitude and the excess pore water pressure are constant ; (2) as the excess pore water pressure increases, the modulus of elasticity decreases much more than the coefficient of viscosity ; (3) the contribution of the elastic force to the horizontal subgrade reaction decreases with an increase in the excess pore water pressure.