PURPOSE: To increase the suction amt. of the fluid to be treated and to increase the treating capacity of an ejector device which sucks the fluid to be treated by the ejection of a driving liquid by forming a nozzle to a toric shape to eject the liquid into an annular shape.

CONSTITUTION: The nozzle 22 is disposed in a large-diameter part 20 of a flow passage 10 and a pipe 24 connecting thereto is connected partly through the large-diameter part 20 to a driving liquid pump. Since the nozzle 22 is made into the toric shape, the driving liquid has approximately the annular shape in the flow passage 10. The driving liquid 200 instantaneously stagnates once in a reservoir part 22a from a pipe 24 and is then discharged by the pressurizing force from a ring slit part 22b into the passage 10 under the pressure uniform over nearly the entire circumference when the pump is driven. As a result, the fluid 100 to be treated is effectively sucked and the driving liquid ejected to the annular shape on the down stream of the nozzle 22, i.e., the cleaning liquid is efficiently mixed with the waste liquid which is the fluid to be treated.