To provide a jig for assembling a small inner-pipe section for transmission, by which a coil case with a flange in a connecting means in the internal cylinder of an excavation rod and the small inner-pipe section for transmission are clamped and fixed accurately and simply and no deviation is also generated even in firm clamping by an upper locknut.

The jig 60 for assembly has a reference fixing member 80 fixed at one-end reference place on a base plane and an adjusting fixing member 70 fastened at the other end in a place adjustable manner. A plurality of boltholes corresponding to boltholes in the connecting member 48 of the small inner-pipe section 40 for transmission are formed to the vertical plate 82 of the reference fixing member 80, and a plurality of detent pins engaged with the end section of the curved slot of a flange 34 in a connecting means screwed with the screw section of a small inner pipe are fitted to the vertical plate 72 of the adjusting fixing member 70. Projecting bodies for positioning are formed at places corresponding to the above-mentioned boltholes in the connecting member 48 in the vertical plate 82 of the reference fixing member 80, and projected positioning retaining sections are formed on the upper sides of the projecting bodies.