To align the axial center of the operating shaft of a sensor with that of a shaft which integrally rotates with a contact segment with high accuracy.

The operating shaft 28A of a sensor 28 is coupled with a rotating shaft 24 which integrally rotates with a contact segment 26 and a supporting member 47 for stopping rotation coupled with the main body 28B of the sensor 28 is retained by a fixed system through a retaining mechanism 48. The retaining mechanism 48 has such flexibility that the mechanism 48 only allows the displacement of the supporting member 47 in the direction connecting the position of the axial center of the rotating shaft 24 when viewed from the direction along the axial center of the shaft 24 and the member of the fixed system at the retaining position.