To obtain a steam ejector capable of reducing the fluctuation of steam pressure supplied from a discharge pipe and detecting the pH of supplied steam.

A high pressure driving steam pipe 3 and a low pressure suction steam pipe 4 are connected to a suction chamber 1 of a steam ejector 10. A pressure sensor 7 and a pH detecting means 8 are attached to a discharge pipe 6. A pressure reducing valve 12 is attached to a by-pass route 11 provided parallel to the steam ejector 10 as a pressure control valve. When the pressure of the steam in the discharge port 6 side decreases to a prescribed value, the pressure of steam in the discharge pipe 6 side is kept in a prescribed range by automatically opening the pressure reducing valve 12 to supply high pressure steam from the driving steam pipe 3 to the discharge pipe 6.