This paper describes a reactor for multi-step syntheses via intermediates with improved selectivity of desired products. This reactor is referred as assembled reactor. This reactor consists of mixing, residence time and heat exchange units. Connection of these units forms a module for a single reaction. Repeated joints of modules enable the reactor to be used in multi-step syntheses. Mixing and heat transfer rate of the assembled reactor was 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 temperature difference of 20-40 K. This reactor was also applied to the synthesis of bromobenzene via p-bromophenyllithium and gave the product yield more than 80%.