Abstract: Diclosed are a fluid mixing element in which at least one helical shaft provided with at least one helical groove on an outer peripheral wall thereof throughtout its length is inserted in a cylindrical passage tube provided with at least one helical groove on an inner peripheral wall thereof throughout its length.According to the mixing element of the present invention, the fluid supplied into the mixing element flows partly along the helical groove formed in the passage tube and partly along the helical groove formed on the helical shaft to produce the turbulent mixing of the fluid in the mixing element. When the fluid flows through the helical grooves formed in the passage tube and on the helical shaft of the mixing element, the phase transfer is also carried out at planes perpendicular to the flow direction by inertia of the fluid. Accordingly, the fluid in contact with the helical grooves and the fluid out of contact therewith are replaced with each other in series.