Micro total analysis system, or µTAS, is a promising tiny analysis system for medical and biochemical analysis. Droplet µTAS in particular handles samples in droplet state, resulting in reducing the amount of sample volume and the inspection time. Therefore, this study aims to realize a novel but simple droplets mixing device for droplet µTAS using electro-conjugate fluid (ECF). The ECF is a kind of functional fluid suitable for micro fluidic devices, which produces a jet flow under a non-uniform electric field applied. This study introduces this attractive fluid to realize a rotational flow which is generated in a tiny cylindrical chamber. When droplets are instilled to the ECF rotational flow, they may be subjected to the centripetal force due to density difference between droplets and ECF, resulting in droplets move to the center of the chamber and mix each other. According to this concept, we first tested cytotoxicity of the ECF, and then we designed an electrode shape required to generate the ECF rotational flow and fabricated a prototype of the mixing device. Furthermore, we optimized the rotational speed of the rotational flow for droplets mixing. As a result, the ECF rotational flow with 6 rps to 8 rps could move 0.3 µl water droplets to the center of φ18 mm chamber. Finally, we conducted a mixing experiment of two droplets and proved that the two droplets mixed in 3.96 s, without any complex controller. According to the cytotoxicity test, this mixing time was short enough for biological samples not to be influenced by the ECF.