When a lot of drift woods due to locally heavy rain flow into a mountain stream, a bridge is caused by serious damages. Damed-up drift woods may cause a flood, and piers and slabs of a bridge can be broken due to the collision with drift woods. Because such damages due to drift woods are recently increasing, it is necessary to consider the prevention against them. In order to make effective guidelines of prevention works, detailed mechanism should be examined. However, 3D numerical simulations of dam-up process of drift woods and resultant overflow around a bridge have not been carried out before. In this study, a numerical analysis by the MPS method is executed in the 3D field to simulate dam-up process of drift woods onto a small slab bridge in a mountain stream.