We have developed a new assembly for the shock compaction of difficult- to-consolidate powder such as intermetal compounds or ceramic materials. We have successfully obtained crack free bulks of consolidated sample from several kinds of powders for example TiAl alloy or Si3N4 powder etc, using this assembly. This assembly consists of three parts. They are, an explosive container, a water chamber and a powder container. Once an explosive is detonated in the container, the detonation wave occurs and propagates to impinge on the water in the water chamber. When the detonation wave impinges on the water, an underwater shock wave occurs immediately. The underwater shock wave interacts with the solid wall in the chamber, converging its strength as it propagates down stream. Finally, the converged underwater shock wave enters into the powder container to consolidate the powder. The mechanism of this process described above has not yet been fully analyzed. In this paper, the interaction process between the detonating wave and the water are investigated by the streak photograph taken by an image converter camera. The underwater shock wave is accelerated in a short time up to its final velocity, which is slightly larger than that determined by the impedance match method. The converging process of the underwater shock wave was also understood by the high speed photograph obtained by framing photographs using shadowgraph system. The incident shock wave comes up to about 18 GPa after being converged by the wall of the pressure chamber. Finally, we suggest the most suitable design of pressure assembly.