PURPOSE: To obtain a jig which facilitates a fine adjustment of centering by oscillating respective hemispheres through the slanting operation of an operation lever and displacing a movable member to or from a fixed member, and making the center of rotation of a driven-side rotating part coincident with that of a driving-side rotating part.

CONSTITUTION: When the assembling operation lever 34 is slanted, the movable member 21, i.e. rotating drum 2 is moved radially to its rotating shaft 4 and aligned thereto. Then when the rotating drum 2 is aligned actually, a measuring element 43 equipped with an electric micrometer 42 is brought into contact with the outer circumferential surface 2B of said rotating drum 2. The rotating drum 2 is rotated firstly and the initial oscillation extent of the rotating drum 2 is measured. When the center of the rotating drum 2 is aligned to the center of said rotating shaft 4, a screw 7 is tightned up strongly to fix the rotating drum 2 to a drum fitting member 5 and the centering adjustment of the rotating drum 2 is completed.