The present study aimed to clarify the influence of the driftwood collision to the destruction of the concrete slab due to tsunamis. The applicability of the empirical formula based on Hertz theory was examined by the large scale collision experiments on the concrete slab with timber in the air and driftwood due to tsunami. The duration time calculated by the formula was good agreement with that obtained from the experimental record of the strain gauge when colliding. It indicated that the impulsive force due to drift wood collision could be estimated by using this formula. Moreover, it was clarified that the destructive force on the concrete slab increased by the collision of driftwood compared with only tsunamis.