In microreactors, which comprise extremely narrow channels, a fluid is in laminar flow. The flow is constant spatially and temporally, and therefore, reactant concentrations are defined spatially, resulting in constant reactions. Consequently, the concentrations of reaction intermediates generated in microreactors are also defined spatially and are always constant. Regulation of the concentrations of the reaction intermediates enables efficient control of the reactions. In the present study, we have developed a basic technique for reaction control in microreactors by Raman microscopic spectrometry, and showed that the concentrations of the reaction intermediates are spatially defined as unique values that are dependent on the reactant concentration.