A new liquid-liquid injection technique with a rigid swirling water flow and a single shot photograph was developed to make observation of a fuel-air mixing process, and then to understand its characteristic quantitatively. In this technique, a liquid fuel was injected into an other liquid to simulate the process of diffusion in a piston cavity of an actual engine. This experiment has been made employing four different configurations of plastic piston cavities; dish, troidal, bowl and square type. These cavities were filled with water and rotated to generate the rigid swirling water flow. Observations from the bottom and side view indicated that the mixing process in the square configuration markedly depends on the orientation of the spray, due to both effects of the swirl and the impingement of the spray on the side wall. The technique is a potential to visually understand the mixing process.