A hollow-cylindrical portion (26) inserted in a hollow-cylindrical portion (16) of one rim section (12) is provided on the other rim section (13). To form a split rim (11), part (head portion) of the inner hollow-cylindrical portion (26) overlaid on the outer hollow-cylindrical portion (16) is radially outwardly expanded by being elastically deformed by a tapered surface (39) of a piston (38) such that the inner and outer hollow-cylindrical portions (16, 26) are in close contact with each other. As a result, relative positions of the rim sections (12, 13) are strictly defined, and this enables high assembling accuracy, particularly coaxiality with a high level, to be achieved. A fine fitted to the split rim (11) can be inspected with high accuracy.