A method for generating an underliquid shock pressure comprises the steps of igniting a combustible mixture gas to form detonation waves, imploding the detonation waves in their advancing direction, propagating a pressure obtained by imploding the detonation waves to a liquid, and converting the pressure to an underliquid pressure which is applied to a workpiece. The pressure obtained by imploding the detonation waves may be applied directly to an elastic rubber body, thus converting the pressure to an underelastomer shock pressure, which is then applied to the workpiece via the elastic body. An apparatus for generating an underliquid shock pressure comprises a combustion chamber whose cross-sectional area decreases from one end toward the other end of the chamber, an ignition chamber to which fuel is fed and in which a spark plug is positioned, a plurality of guidance paths extending from the ignition chamber and communicating with one end of the combustion chamber, each of the guidance paths having substantially the same length, and a pressure medium chamber connected to an opening of the other end of the combustion chamber having the smallest cross-sectional area. The pressure at the smallest cross-sectional area end of the combustion chamber is applied to a liquid to convert the pressure into underliquid shock pressure, which is then applied to a workpiece either directly or via an elastic body interposed between the liquid and the workpiece.