As a first step to clarify the synergistic extraction mechanism of gallium with the mixed extractant of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester and 5-chloro-8-quinolinol, the initial extraction rate of gallium with 5-chloro-8-quinolinol alone was measured using a stirred transfer cell. As a result, the rate process was found to be limited by a diffusion accompanied by the formation reaction of chelating complex in the aqueous solution. The calculated results based on the model generally explained the experimental data.