Unsteady vortex structure in the wake behind bluff body was experimentally investigated by wind tunnel experiment.A flexible Cyclist Accessories - Helmets - Full Face rectangular plate, which was made of a soft polyurethane block, shows the sway motion and flow-induced vibration caused by fluid force.Phase-averaged vortex structures behind the flexible plate were educed by Particle Image Velocimetry (PIV) using light sensors.The size of recirculation region and upwash Blood Sugar Support flow in the wake were influenced by unsteady motion of the plate vibration.