To clarify the mechanism of observed violent vibration of a brace adjacent to a high circular stack, an experimental study was performed. The experimental apparatus consists of a vertical elastic rod placed close to a large solid cylinder. Accordingly, airflow makes the rod whirl at its first natural angular frequency. Whirling motion grows as airflow velocity increases. Whirling direction depends on the relative location to the cylinder and not on airflow velocity. Further, analytical study was done by assuming the flow past cylinder is frictionless, irrotational and incompressible. Analytical results show fairly good agreement with experimental ones as well as the actual brace vibration.