PURPOSE: To efficiently react a reaction solution separated into two phases by reaction by simple operation using a small-sized reactor having a simple structure.

CONSTITUTION: A reactor 1 is constituted of a hollow casing 1 and the mixing element 5 arranged in the hollow casing 1. The mixing element 5 has spiral elements 6 composed of right-handed blades alternately provided at an angle of 90° and spiral elements 7 composed of left-handed blades. The reaction solution supplied to the reactor is successively divided by the spiral blades 6,7 and, therefore, the contact efficiency and reaction efficiency of the divided reaction solutions are enhanced in the latter reaction period. The reaction solution is supplied to the reactor under such a condition that means linear velocity is 0.15-1.5m/sec and the Weber number related to the diameter of a liquid droplet is 30-900.