Reevaluation of earthquake load to pile foundation and it's design method is strongly suggested from the evidence of damage in Sendai City area caused by the Off-Miyagi Prefecture earthquake in 1978. This is the first experience in Japan that the piles used as building foundations placed in horizontal soil deposits being damaged by vibrational effects of earthquake ground motion. Several examples of the damaged piles and response analysis of an example of damaged pile foundation are introduced and the causes of the damage are discussed. And then, participation factor of horizontal force applied to piles is discussed for the purpose of rational seismic design of pile foundations based on earthquake observation data of a fourteen storied residential building supported on long piles and vibrational tests by shaking table for model ground-pile-building system.A fundamental idea of design of pile foundation is proposed lastly as an appropriate statical method in which seismic effects are introduced.