We have applied our microreactors providing better mixing performance at lower flow rates to the Sandmeyer reaction of aniline via an unstable diazonium ion. The mixing performance of our microreactor was evaluated using the well-known Villermaux-Dushman reaction. Our microreactor provided better mixing performance than the IMM (Institute fur Mikrotechnik Mainz GmbH) single mixer even at low flow rates of less than 8 mL/min (8 x 10(-6) m(3)/min). With using our microreactor, the mixing time became shorter and the reaction time for each stage in multistage reactions could be put into a more proper perspective. In the Sandmeyer reaction, the maximum yield of the objective product, chlorobenzene was 80% when the times for the 1st and 2nd steps were 23.6 and 179.9 s, respectively. We also performed reaction rate analyses to understand the reaction mechanism. The calculated results for aniline, chlorobenzene, and the byproduct from chlorobenzene were in good agreement with the experimentally measured ones. It was suggested that reaction rate analyses for experimental data using microreactors are effective for understanding reaction conditions in multistage reactions. The yield in this study was 29% higher than the value reported for the batch method and 9% higher than that for the chip-based method. In this study, with using our two microreactors providing fast mixing in series, an appropriate reaction time and reaction temperature were set for both the 1st and 2nd steps, while there were micro channels for both steps on a plate in the chip-based method.