The transition between regular and Mach reflection of an oblique shock wave has been investigated by many researchers for steady, pseudo-steady and unsteady Flows, However the problem for unsteady Flows such as the reflection of a shock wave at cylindrical concave or convex walls has not been fully understood. The problem considered in this report is that of transition from Mach to regular reflection of a shock wave at a concave wall. Experimental investigations have been performed using a model concave wall. The wall is composed of two straight walls, and the second wall in more inclined than the first one. The process of shock reflection at this wall has been observed in detail, and the transition criterion from Mach to regular reflection in this case has been presented experimentally.