The problem, considered in this report, is that of transition from Mach to regular reflection of a shock wave over a concave wall. Experimental investigations have been performed using a model of concave wall. The wall is composed of two straight sections, and the downstream second section is more inclined than the upstream first one. The process of shock refection over this wall has been observed in detail, and the transition criterion from Mach to regular reflection in this case has been presented experimentally.