Assuming a simple mixing section built into a small-scale flow reactor operated mainly under laminar flow conditions, we investigated the mixing characteristics in a co-current double-tube mixing section with an orifice plate. The mixing characteristics of two liquids in the mixing part were evaluated by the segregation factor in the Villermaux. Dushman reaction for flows in the region where Reynolds number was 4000 or less and the flow ratio of the annular region to the inner pipe was 100 : 1. The parameters examined were the orifice diameter and the total flow rate of the fluid (sum of the flow rate from the annular part and the inner pipe). It was found that by installing an orifice plate just downstream of the inner pipe outlet, extremely good two-liquid mixing characteristics were exhibited even under laminar flow conditions. The smaller the orifice diameter was, the better were the mixing characteristics. In addition, the mixing characteristics improved with increasing total flow rate. In order to investigate the reason for the improvement of mixing characteristics by installing an orifice plate, water colored with pigment was supplied from the inner pipe under the same conditions as in the mixing characteristics evaluation experiment, and the flow state in the pipe was visualized. It was found that the vortex from the shear layer formed at the boundary between the jet from the orifice and the surrounding fluid and the turbulence of the jet itself contributed greatly to the mixing characteristics. It was clarified that mixing performance comparable to existing micromixers could be obtained depending on the conditions.