A transient flow induced by the shock front impinging on the four kinds of Laval nozzles, which were installed in the low pressure side of a shock tube, was observed optically by the schlieren method, and the starting process of nozzle and the value of initial diaphragm pressure ratio required to start the nozzle have been clarified. The nozzle starting time the duration of flow with constant Mach number were determined experimentally, as a function of the nozzle Mach number and the initial diaphragm pressure ratio. Moreover, it was found that the reflection of slip-stream caused by the Mach reflection of a shock wave is classified into two essentially different patterns of regular and irregular types. This phenomenon is similar to the two patterns of regular and Mach reflections occurring in the reflection of shock wave.