PURPOSE: To enhance mixing efficiency by providing a fluid passage structure in a passage pipe so as to form a plurality of fluid passages and providing an auxiliary body magnifying a fluid contact area and shearing force to the inner peripheral surface of the passage pipe and the surface of the fluid passage structure.

CONSTITUTION: The mixing element is constituted of passage pipes 3, 8 through which a fluid flows and fluid passage structures 6, 11 forming a plurality fluid passages in said passage pipes. Auxiliary bodies 7, 12 (e.g., porous bodies), which magnify the fluid contact surfaces of the passage pipes 3, 8 and the aforementioned fluid passage structures and shearing force, are provided to the inner peripheral surfaces of the pipes 3, 8 and the surfaces of the structures 6, 11. Each of the aforementioned fluid passage structures is composed of the spiral blade partitioning the inside part of each of the aforementioned passage pipes to form a plurality of the fluid passages and twisted from one end part in the longitudinal direction thereof to the other end part thereof in a right or left direction. A plurality of mixing elements thus constituted are connected in the longitudinal direction of the passage pipes so that the adjacent end edges of the blades of the adjacent elements form predetermined angles mutually.