An auxiliary instrument for fixing a rod (61) places a detent pin on a head portion of the screw. The auxiliary instrument (61) comprises an inner cylinder (63), an outer cylinder (67), a first lever (73A) and a second lever (73B). The inner cylinder (63) has a plurality of claw portions (65, 65) and an opening employed to introduce the detent pin into an interior thereof. The outer cylinder (67) is arranged to surround the inner cylinder (63) and has a claw pressing portion. The first lever (73A) is rotatably connected to the inner cylinder (63) and arranged in substantially perpendicular to an axial direction of the inner cylinder (63). The second lever (73B) is rotatably connected to the fist lever (73A) and the outer cylinder (67) and arranged in substantially perpendicular to an axial direction of the outer cylinder (67). When the first and second levers (73A, 73B) are operated, the claw portions (65, 65) are pressed inward by the claw pressing portion to hold the head portion of the screw.
