In this study, we developed a customized low cost and low energy bubble generator that can control bubble size. Hence, it can be used not only in the water treatment process but also in various other processes. This device was able to generate bubbles with a very simple system using only a general pump and a mixing chamber. Increasing the number of partition walls in the mixing chamber reduced the bubble size. Furthermore, bubbles of a few hundred nanometers were produced by the shear stress caused by increasing the thickness of the partition wall. Although the generated sub-micron bubbles were too small for their exact size to be measured using an image analysis and particle counting method, it was possible to confirm their existence indirectly through the coalescence arising from ultrasonic irradiation. The device used in this research is simple and allows bubble size to be adjusted easily by controlling the design of the mixing chamber. Therefore, it can be applied to a water treatment process, as well as a variety of other processes.