There have been no investigations of the transient characteristics at starting of supersonic air ejector. In the present paper the measuring methods and results of transient characteristics are discussed. The experiments were carried out as follows : After a solenoid valve, which was installed before the supersonic nozzle, was suddenly opened, the pressure changes of the driving air, the entrained air and the mixed air with time were measured with an automatic measuring system, which consisted of semi-conductor type pressure sensors, amplifiers, 8ch-A/D converter and a personal computer. The transient characteristics were measured in the three cases of all closed system, secondary-opened system and all opened system. They were discussed with the relations of flow rates, accumulated flows, pressure ratios to time. One of the most interested results is that the transient characteristics especially in the case of the all opened system shows the behaviors as first-order delay system.