Instability of the three-dimensional boundary layer on a yawed circular cylinder is investigated experimentally. An unsteady disturbance is artificially introduced from a point source on the cylinder to clarify instability characteristics of the flow. Streamline-curvature instability is confirmed to dominate rather than cross-flow instability in a certain range of Reynolds number in agreement with the prediction of linear stability theory, although the amplitude is modulated by the effect of stationary mode of cross-flow instability.