A cylinder-shaped flow passage in which a first fluid flows includes an internal cylinder which is smaller in diameter than the flow passage. A swirl-generating stator having four vanes is radially fixed in the internal cylinder. A header space for supplying a second fluid is provided to the outer circumference of a wall surface of the internal cylinder in contact with flow separation areas which are formed along downstream surfaces of the swirl-generating stator as the first fluid runs into the swirl-generating stator. The wall surface of the internal cylinder is formed with openings through which the flow separation areas communicate with the header space. The second fluid supplied into the header space flows through the openings into the flow separation areas, and is diffused along the vanes of the swirl-generating stator to be swirled and mixed into the first fluid applied with swirling force by the swirl-generating stator.