To heighten mixing efficiency of waste water and a treating agent to accelerate their reaction as fast as possible.

This mixer is provided with an upper chamber 11 to which waste water is fed and a lower chamber 12 into which the waste water is made to flow through the upper chamber 11. The upper chamber 11 is provided with a cylindrical of runnel-shaped falling fluid cylinder 26 into which the waste water is made to overflow from the outside to the inside, a dispersing cone 17 having a conic outer peripheral surface to be made to rotate in the posture in which the top thereof is turned upward in the falling fluid cylinder 26 and a feeding part 16 for feeding a treating agent toward the dispersing cone 17. Then, an agitating and mixing mechanism 18 in the lower chamber 12 agitates, shears and mixes the fluid to be treated to which the treating agent is made to add.