A new shock consolidation technique applying converged underwater shock waves was proposed and tested for difficult-to-consolidate powders such as ceramics and cBN powders. We obtained bulks with fewer cracks than those obtained by other shock consolidating techniques such as the double-tube method. We present here the numerical simulations of the convergence of underwater shock waves to obtain ultra-high pressure of over 17 GPa using this technique. The behaviors of the underwater shock waves very near the explosives were clarified. The effects of the deformations of the pressure vessel in this technique are also mentioned. The velocity of the underwater shock wave is so high that the effects of deformation of the pressure vessel are quite remarkable.