An ejector includes a nozzle, a suction chamber, and a diffuser. A diffuser further includes an attachment configured to change the dimensions of the outlet flow path. The attachment changes the length X of a narrowed flow path and the length Y and the inner diameter D of a parallel flow path such that expressions (1) and (2) are satisfied:
X=A×D  (1),
and
Y=B×D  (2).

An ejector includes a nozzle, a suction chamber, and a diffuser. An outlet flow path includes a narrowed flow path having a first tapered surface narrowed toward downstream, a parallel flow path having a constant sectional area, and a parallel flow path having a second tapered surface expanded toward downstream. The diffuser further includes an attachment configured to change the dimensions of the outlet flow path. The attachment changes the dimensions of the outlet flow path such that the ratio of the tapered angle of the first tapered surface to the tapered angle of the second tapered surface is higher as the sectional area, i.e., the inner diameter, of the parallel flow path is smaller.

An ejector includes a nozzle, a suction chamber, and a diffuser. A diffuser further includes an attachment configured to change the dimensions of the outlet flow path. The attachment changes the length X of a narrowed flow path and the length Y and the inner diameter D of a parallel flow path such that expressions (1) and (2) are satisfied:

X=AD(1),

and

Y=BD(2).