Flow-induced in-line oscillation of tandem circular cylinders has been experimentally studied by free-oscillation tests in a wind tunnel. The upstream cylinder model with the reduced mass-damping parameter Cn=0.99 was supported by eight coil springs to be moved in the in-line direction, and the downstream cylinder model was fixed. The gap between the upstream and downstream cylinders was changed from s/D=0.3 to 2.0. We measured the responses of the model in in-line oscillation and the vortex-shedding frequency. The flow around tandem circular cylinders was visualized by the smoke-wire method. For the gap ratios of 0.3≤s/D≤1.5, the single wide excitation region appears, and the exciting oscillation region extends up to reduce velocity of Vr=2.7. The oscillation characteristic is similar to the oscillation response of the single circular cylinder with a splitter plate. The oscillation of the excitation region is mainly induced by the symmetry vortex shedding. The upstream circular cylinder with the gap ratios of s/D≥2 has two kinds of exciting oscillation mechanisms, similar to that of the single circular cylinder. The vortex shedding during the in-line oscillation shows symmetric and antisymmetric patterns in the two excitation regions, respectively
