This paper describes a reactor for multistep syntheses via intermediates with improved selectivity of the desired product. This reactor is referred to as an assembled reactor. It consists of mixing, residence time, and heat exchange units. Connection of these units forms a module for a single reaction. Repeated joints of these modules enable the reactor to be used in multistep syntheses. Mixing and the heat transfer rate of the assembled reactor were evaluated. This reactor enabled rapid mixing under the total flow rates of more than 30 mL/min and heat transfer efficiency of the order of 10 K per 10 ms in the heat exchange of fluids with a temperature difference of 20-40 K. This reactor was also applied to the synthesis of bromobenzene via p-bromophenyllithium and gave a product yield of more than 80%.