Appropriate patterning of pH distribution to obtain high conversion and selectivity of the main product was analyzed for series-parallel reactions whose reaction rates strongly depend on hydrogen ion concentration, under laminar flow in microchannels. When pH distribution in the inlet solution is adjusted so as to enhance the main reaction and suppress side reactions, the selectivity increases but conversion decreases. However, by increasing the number of inlet channels, it is possible to raise both conversion and selectivity. In addition, the conditions for obtaining high selectivity by collecting only the partial stream containing a high concentration of the main product at the Outlet of a microchannel were analyzed.