Shock absorber provided with a piston and a cylinder, both secured to a part of a vehicle. A nonreturn valve is present in either the piston or the base of the cylinder. The nonreturn valve includes a valve plate and a valve seat plate, both annular. Near the center, the valve plate is arranged at a distance from the valve seat plate. At low flow velocity, elasticity of the valve plate keeps it at a distance from the valve seat plate, allowing unimpeded flow. As flow increases, the valve plate deforms elastically, gradually closing off a diametral opening in the valve seat plate. An opening remains between the valve plate and the valve seat plate at the spacer. As the pressure difference across the nonreturn valve increases, the valve plate closes off with respect to the valve seat plate over the entire circumference. This movement takes place as a wave.