To prevent a diaphragm from deforming by welding heat, to prevent deterioration of workability caused by operation for correcting thermal deformation in one arc and two arcs, and further in the case of two arcs, to prevent deterioration of weldability on account of large deviation of a weld line caused by bias (transfer) in the longitudinal direction of a core.

A positioner face plate fixes, on a positioner, a core composed of a tack welded square steel tube 1 and a diaphragm 2. This positioner face plate includes: a supporting plate 21 that is rotatably driven; a plurality of clamping members 22 that are arranged on a core restraining face of the supporting plate 21, that clamps a plurality of places of the diaphragm 2, and that supports the diaphragm 2 with a prescribed space apart relative to the restraining face; and a plurality of blocks 23 that are arranged on the core restraining face of the supporting plate 21 in a manner interposed between the supporting plate 21 and the diaphragm 2 and that have a dimension same as or smaller than the prescribed space.