We present a simple microfluidic method to generate high-density femotoliter-sized microreactor arrays within microfluidic channels. In general, we designed a main channel with many small chambers built into its walls. After sequentially infusing aqueous solution and organic solvent from a single tube into the device, aqueous droplets are confined in the chambers by the solvent flow. The generated reactors are small and stable enough for carrying out ultrasensitive biochemical assays at single molecule levels. As a demonstration, in this paper, we optically observed hydrolysis activity of β-galactosidase enzymatic molecules in the reactor arrays at single molecule levels. Further, this method has the following advantages: (1) the droplets are observable immediately after formation and (2) its simple procedure is sufficiently robust such that even handy infusion of the preloaded solutions is reproducible. We believe our method provides a platform attractive to a variety of single molecule studies and sensing applications such as clinical diagnostics.