In a microreactor in which a plurality of fluids L1 and L2 are led through their respective feed channels to merge in one reaction channel and react with each other while circulating, a spiral reaction channel is formed by cutting a spiral screw thread on either the outer surface of a round-bar-shaped core member or the inner surface having a circular cross section of an outer cylindrical member and engaging the outer surface of the core member in close contact with the inner surface of the outer cylindrical member.