Abstract: An apparatus for generating shock waves by means of a supersonic projectile to effect, for example, phase transformation of a substance. A hammer in a cylinder is accelerated by introduction of compressed air into the cylinder, whereby a plunger formed integrally with the hammer at one end thereof is thrust into a hydraulic pressure chamber filled with a liquid and disposed coaxially with the cylinder. Thereupon, a high hydraulic pressure is generated in a hydraulic pressure chamber, and this hydraulic pressure accelerates a piston in a piston chamber connected with the hydraulic pressure chamber. The piston in turn compresses a light gas adiabatically to a high pressure, and a projectile fitted at one end of a barrel is thrust forward under the force of this high-pressure gas.