Reuse of existing foundations can be economical and ecologically beneficial when a structure is rebuilt. However, old underground piles are not reused because the new foundation plan is different or the piles lack sufficient bearing capacity. In this study, a numerical analysis is performed to determine the foundation input motion during an earthquake for new pile foundations in the case where old piles have been left underground. Dynamic soil-pile interactions and pile group effects such as wave propagation from adjacent piles are taken into consideration. The analysis is carried out with the aim of revealing the influences of the diameter, number and layout of the unused piles on the foundation input motion. The results indicate that when the diameter of the existing piles is large, and the new pile foundation is constructed in close proximity to them, the foundation input motion is larger than in the case where there are no existing piles by frequency. Furthermore, when the existing piles have a nonuniform layout, twisting of the new foundation can occur.