The purpose of this study is to investigate flow pattern around a mosquito's wing while it is beating. Reynolds number and Strouhal number when mosquito was flying were calculated by an analysis of pictures from a high speed video camera and a microphotograph. Beating motion showed three types ; a flapping motion in which wing was moved up and down, a lead-lag motion in which wing was moved back and forth, and a feathering motion in which wing was twisted. Those motions were observed using the high speed video camera. An enlarged scale model testing was carried out using a very low speed wind tunnel under the condition that Reynolds number and Strouhal number were set to be equal to those of real mosquito. Phenomena of periodic vortex growth and its release from a wing model moving up and down were visualized with a smoke method. By changing wing's attack angle, vortex pattern was changed. The vortex was analyzed and the circulation was calculated from visualized pictures.