PURPOSE: To have effective conversion of a mixture gas having high velocity energy into pressure energy by inserting two or more perforated plates with baffle effect, or wire meshes, in a spreading pipe at diffuser outlet of a steam ejector.

CONSTITUTION: Two or more perforated plates 4 with baffle effect, or wire meshes 5, are inserted in a spreading pipe 2 of a diffuser of steam ejector to increase the fluid resistance inside the spreader 2, so that the speed of the exhaust gas flow passing through the pipe can be uniformed. Accordingly the condition of blowing off through the central part is eliminated, and speed difference of the exhaust gas passing through the spreading part 2 of the diffuser can be lessened. Further a large effect for converting the velocity energy into pressure energy can be obtained to ensure sufficient restoration of the pressure.