Outer cables disposed in a box girder in the longitudinal direction of a box girder bridge to prestress the box girder each have a transparent sheath made of a transparent polyethylene resin, vinyl chloride resin, polypropylene resin or the like, thereby allowing the grouting condition in the sheath to be inspected reliably and easily. If a colored grout is filled, the filling condition can be inspected even more reliably and easily. In addition, it is possible to readily find a portion left unfilled after the grout has hardened and to refill it for repairing by using openings for injection and ejection made at each side of the void left.