The axial flow of a supersonic high density plasma beam into an asymmetric mirror machine has been investigated experimentally. It has been found that in front of the second mirror peak, two supersonic‐subsonic flow transitions and shock formations occur. The subsequent motion upstream, of the first and stronger shock front has been measured by the time‐of‐flight method through diamagnetic loop and electric double probe signals. The shock front speed has been found to be one third the speed of sound in the neighborhood of the sonic transition and decreases to zero ahead of the upstream mirror.