PURPOSE: To fix a small diameter part to a supporting base with certainty for supporting by forming a part tapered from an end of a large diameter part to a periphery of the small diameter part on a stapped part provided between the large and small diameter parts, and meshing the tapered part with a fixing hole of the supporting base.

CONSTITUTION: A shaft member 15 has a flange part 15A, a large diameter part 15B, a small diameter part 15C, and a plurality of spiral teeth parts 15D on a stepped part between the large diameter part 15B and the small diameter part 15C. The spiral teeth parts 15D are prepared as a bevel gear which has a conical tapered portion tapered from the side of the large diameter part 15B to the periphery of the small diameter part 15C. When assembling as a journalling structure, the spiral teeth parts 15D is each in contact to a peripheral edge of a fixing hole 4 of a supporting base 3. The relation between length L1 of the large diameter part 15B of the shaft member 15 and length L2 obtained by adding an axial length of the spiral teeth parts 15D thereto is set such that the length L1 corresponds to a minimum thickness T1 allowable as a tolerance of a rotational lever 10 and the length L2 corresponds to an allowable maximum thickness T2 of the rotational lever 10.