To detect the cavities formed behind the impeller blades, the electrical resistivity technique was employed in this study, The transition conditions between large cavity and clinging cavity were easily detected by the proposed method and the reliability of the probe was confirmed by the comparison with literature. The characteristics of the cavities were observed as a function of impeller rotational speed and air flow rate. As the air flow rates increase, the type of cavities are changed from clinging cavities to large cavities. Large cavities were not observed in behind the impeller blades at high impeller speeds. In the region of the formation of alternate large and clinging cavities, the shapes of cavities were not changed and the positions of cavities were rarely shifted to other positions.