The mechanism of the onset of the aerodynamic excitation of suspended roof models was investigated mainly in a wind tunnel. It was observed that the separated shear layer on stationary models rolled up in very faint vortices, and that the shear layer was greatly excited by the model motion in a range of wind speed centered around the onset wind speed. The power spectrum of the fluctuating component of wind speed on stationary models was characterized by a broad-band spectrum with a peak. Since the inverse of the obtained Strouhal number was equal to the reduced onset wind speed, it can be said that the role for the occurrence of the excitation of the models is played by the vortices that are formed originally on stationary models, and are greatly amplified by the model motion. It was also found that there is the possibility that vortex excitation occurs for three dimensional models.