The pressure of mud-debris flow acting on an obstacle was evaluated for cases in which the mud-debris flow was regarded as a non-compressible, then as a compressible fluid. In a non-compressible fluid, an equation of the fluid motion describes an equilibrium between pressure and potential rate. For a compressible fluid, the force is calculated by the density, impact velocity, and propagation velocity of pressure in the fluid. Analytical results showed that in a very few times, the force acting on a structure was some times the expected momentum value (per second)