To prevent the occurrence of erroneous assembly in spite of using a common rotor for a rotating speed sensor in an automatic transmission of different variation.

A recessed part 17 is formed at one end side 16d of a rotor boss part 16c of the rotating speed sensor 3. The outer diameter dimension of the recessed part 17 is smaller than the outer diameter of a snap ring 12 and larger than the outer diameter of a projecting part 20a of a collar 20. In the normal fitting attitude (reverse to the figure) of the rotor boss part 16c, the recessed part 17 is fitted to the collar projecting part 20a, but in the erroneously assembled state, the collar projecting part 20a abuts on the other end face 16e of the rotor, and the collar 20 is biased axially backward by that portion to make it impossible to mount the snap ring 15 in a recessed groove 21.