The collapse mechanism has been investigated by using FE analysis. The simulation results showed the similar failure mode to the actual bridge collapse by modeling the corrosion damage correctly. The loss of post-buckling strength by both the rupture of web-lower flange connection and web-vertical stiffener connection at the end span was thought to be the cause of collapse. On the other hand, the section loss of horizontal and vertical stiffeners, and the rupture of lower flange did not contribute significantly. According to the analysis results, the collapse might be avoided by repairing the rupture of web-vertical stiffener connection, since the recovery of shear capacity should be expected by the formation of diagonal tension field and the frame effect.