After the formation of a dense shell in the third phase of evolution of the Supernova Remnants a shock wave moves ahead which compresses the interstellar medium. Velocities of the shell and shock wave are calculated from the relations across these fronts. Interaction of a shock wave with a contact surface of different densities is discussed. It is found that for lower density a rarefaction wave is reflected which results in the acceleration of the shell. For a higher density a shock wave is reflected which falls back on the shell. The possible cause of highly irregular structure of the Supernova Remnants is the passage of such type of wave.