The purpose of this paper is to investigate the effect of a semi-circuler tripping wire on the aerodynamic character of a circuler cylinder as basic research for study on Rain-wind induced vibration. As a results of experimental investigation on the fluid forces acting on a circuler cylinder with a semi-circuler tripping wire, it became evident that there were bi-stable flows in the limiting condition of R_e number and relative height of a trippig wire to diameter of cylinder d/D. In a state of the bi-stable, the mean values of lift forces in short times show rectanguler change intermittently. It is decided the condition convereted into the bi-stable flows by statistic characters of flactuating lift and the change of fluid forces with the switching of two flow patterns by analysis of the measured fluctuation of fluid forces transformed into low frequency fluctuation through the low pass filter with 2 or 3 times period of eddy shedding from a circuler cylinder.