In this study, the impact force when the body with flat bottom runs into flat water surface is treated. In this case, the compressibility of fluid must be always regarded. When approaching velocity is low, the impact force on the body is equal to the momentum given to fluid within area given by the wave equation. On the other hand, when approaching velocity is high, the impact force is equal to compressure stress of fluid just under the body. Under these considerations, two impact force equations are obtained respectively, and the boundary of two states can be defined. And then, parameters ε (x) which is defined by the compressibility of fluid and elasticity of the body, and Me which is defined by the ratio of approaching velocity to the velocity of sound in fluid are substantial quantities which dominate the impact phenomena.