Characteristics of pile forces induced by earthquakes are discussed from the viewpoint of the inertial and kinematic force. In most seismic design codes, pile foundations are designed merely against inertial force. However, soil deformation caused by seismic waves generates curvature of piles and subsequently a bending moment along their whole length. In this paper, the dynamic response of laterally-vibrating pile foundation is investigated through numerical studies on a soil-pile-structure system with different ratios of the natural period of structure (Ts) to that of soil deposit (Tg). Both linear and non-linear seismic analyses have been performed for comparison. Then, a practical method to evaluate the pile force induced by not only inertial response but also by kinematic response is proposed.