Abstract: In the present invention, testing is performed by injecting grout (G) into a full-scale specimen comprising a cable that is fabricated at a place other than a construction site. The cable has a sheath made of a transparent material and has the same three-dimensional configuration as that of an internal cable (2°) at the construction site. Alternatively, grout (G) is injected into a partial specimen comprising a lengthwise part of the cable where an air trap is likely to occur. The testing is performed under a plurality of different testing conditions. The best grouting conditions are selected from testing results and applied to grouting actually carried out at the construction site.