As the first stage of study on the steam ejectors, it is the purpose of this paper to know experimentally the condition of flow and the extent of mixing of steam and inhaled gas (air on this experiment) at each cross-section of the ejector. By the experiment we have obtained the characteristic curves, the relation of vacuum of mixing chamber and weight of inhaled air. And on each characteristic curve, we have found the condition and extent of mixing of flow by means of measuring the static and Pitot pressures on each section of the diffuser. By these results, we have made clear the so-called hysteresis phenomena of steam ejector in which two characteristic curves are present on the same outside conditions, high vacuum and low vacuum characteristics, and shown the difference of the two characteristics.