To omit a lower horizontal structure by laying out a diagonal bracing from a fulcrum area toward an upper flange in a reinforcing structure of an I girder bridge in which cross section I section girders are lined up in the axial direction of a bridge.

In an I girder bridge 20, a diagonal bracing 22 is slantingly provided between a point 2A on the side of a flange 5 and a point 2B on a plate girder 2 between a sway bracing 17 at the fulcrum area and an intermediate sway bracing 18, thereby forming reinforcement structure 21. This sway bracing omits a lower horizontal frame as the reinforcement structure 21 having shear rigidity in the longitudinal direction of a bridge and in the direction at right angles. In the case when a transverse force having an angle Θ acts on the longitudinal direction of the bridge, both the diagonal bracing 22 and the sway bracing 17 on the end work together to a component force at right angles to the longitudinal direction of the bridge while the diagonal bracing 22 resists against the component force in the longitudinal direction of the bridge, which makes it possible to transfer this component force in the direction of a floor slab having a large shear rigidity and resist definitely.