This paper describes seismic response analysis of the Yokohama-Bay Bridge during the 2011 Great East Japan Earthquake, focusing on pounding behavior between tower and girder. In the first part, analysis of recorded seismic response data shows the unexpected response during the main shock and the largest aftershock: 1) the transverse pounding between girder and tower, 2) changes in natural frequency and damping. Second part describes seismic response analysis considering the pounding effect. The results of this numerical analysis reproduce the observed response and show that nonlinear response was caused by the pounding. Finally, we analyzed effect of more severe earthquake excitation. The result of inland level 2 earthquakes indicates the pounding between tower and girder causes the damage of wind shoe.