Stress triaxiality is proposed as one of the key parameters to discuss the cause of brittle fracture during earthquakes in steel structures. This study analytically investigated the features of stress triaxiality change in steel bridge bent subjected to earthquakes. The target structure is a steel bridge bent actually fractured during the South Hyogo prefecture Earthquake. From the investigations, it was confirmed that the triaxiality change was not affected significantly with each cycle of corner deformation during earthquakes and with the stress-strain curve of steels. The triaxiality change depends on the geometry of weld bead at fracture origin.