Impact pressures due to breaking wave sometimes caused serious damages on a ship or marine structures. Impact pressure exerted on the face of a flat plate by a breaking mass was studied experimentally and analytically. Experiments have been carried out with a free-falling water mass apparatus. Recorded time history of impact pressures can be categorized into two typical profiles. One is an impulsive peak with a damped oscillation and the other is without the oscillation. The damped oscillation can be explained by a well-known theory with a trapped air pocket between the breaking mass and the plate. And our analytical model with a curved water surface reveals that a change in momentum due to added mass on the plate yields impulsive force.