A rubber specimen-stretching jig including a jig body has a disk-shaped pedestal; a columnar rubber specimen-fitting support erected on the pedestal; and a columnar retaining ring-fitting support projected from an upper end surface of the rubber specimen-fitting support; and an annular retaining ring removably fixedly fitted around the retaining ring-fitting support.The jig body is inserted into the cylindrical rotor, with one or more stretched annular rubber specimens wound around the rubber specimen-fitting support of the jig body; and with an inner surface of the rotor fixedly pressed against an outer surface of the pedestal and that of the retaining ring and brought into contact with the annular rubber specimens, a solid NMR of each of the annular rubber specimens is measured while the stretched annular rubber specimens are being rotated at a high speed by interlocking the stretched annular rubber specimens to the rotor rotated at a high speed.