PURPOSE: To enhance the breaking-proof stress of a major cross-beam by fixing an outside cable in which a pre-stress is introduced in the major cross-beam of a bridge in its longitudinal direction to the major cross-beam over a predetermined length near the middle part and by effectively acting the pre-stress.

CONSTITUTION: An outside cable 4 in which a pre-stress is introduced in the major cross-beam 1 of concrete in its longitudinal direction is fixed to the major cross-beam 1 at least one place at nearby the intermediate portion. Execution is done so that installation intervals of fixing devices 17 may be about 40% each span, L1 and L2. In introducing the pre-stress in the outside cable 4, external force is allowed to act on the main cross-beam at the intermediate portion as well as both ends to give almost the same distortion as that of the major cross-beam 1 to the outside cable 4. The result enhances the breaking proof stress of the major cross-beam against a variable load and an impact load.