To facilitate the centering of butt members by, in a centering jig used for joining raw material such as a steel pipe and an alloy pipe, severally providing both end parts with a locking part for drawing, and providing two connecting members about in parallel and a plurality of rigid bridging members which are ladder-shapedly erected at proper intervals between connecting members.

A centering jig 15 for ladder-shaped butt members is composed by setting the ring of a connecting member 6 between hook plates provided in a pair at both the ends of each of rigid bridging members 7 located in parallel at proper intervals and fastening both the hook plates. In the case where steel pipes 3 are butt- joined, after both the steel pipes 3 are placed opposite on a frame, a centering jig 15 is wound around the steel pipes 3, and drawing means 16 are locked on drawing lock parts at both the ends of the connecting member 6. Next when both connecting members 6 are drawn by the drawing means, the axial centers of both the steel pipes 3 are made to mutually coincide by each rigid bridging member 7 and centered. After that, a butted part between both the steel pipes 3 is temporarily welded, and then the centering jig 15 is removed, and then the butted part is permanently welded.