We have developed a sliding valve system that can generate a slug flow in the microchannel of a microchemical reactor, in which the size of slug segments can be controlled by adjusting the sliding valve frequency. The slug flow system composed of two immiscible liquids has hitherto been thought to be effective mainly for physical diffusion processes such as extraction operations. In this study, we have tried to extend the application to chemical reactions, especially the rapid inorganic neutralization reaction between an acidic metal ion solution and an alkaline solution. Here we proposed a new reaction model: a reaction occurring in slug flow in the microreactor channel is a result "mixing" between reactant segments in a series of "micro-beakers" generated by sliding valve movement. Based on this model, the sliding valve frequency can control the size of the segmented slug, which is equivalent to the size of the micro-beaker, and ultimately the nature of the reaction. We report the experimental result of synthesis of a transparent nanocolloid particle sol, phosphorus-containing tin oxide (PTO), using the sliding valve system.