The interaction of a plasma flow with a mirror field is studied by changing the mean free path of ion-ion collisions in the BSG-IA device. In the case of a collision-dominated plasma, it has been shown in the BSG-I and II devices that a shock occurs when a supersonic plasma flow encounters a mirror field. When the mean free path is made longer than the characteristic length of the mirror field, shock effects no longer occur. The diamagnetic signal in front of the mirror still increases compared with that obtained in the case without a mirror. A particle orbit model given by Gilleo for the interaction of a plasma flow and a mirror field is refined and the increment of the diamagnetic signal explained by the refined model.