changes with a frequency proportional to the rpm of the hub 3 the principle that the change in voltage generated in the coil 38 corresponds to the change in density of the magnetic flux flowing in the stator 37 is the same as that widely used in the prior art rpm sensor and therefore an explanation of it is omitted the reason that the direction of flow of the magnetic flux flowing in the stator 37 changes in response to the rotation of the tone wheel 29 will be explained below as described above the pitch of the rotating notches 33 located around the tone wheel 29 is the same as the pitch of a pair of poles pair of south poles or a pair of north poles of the south poles and north poles alternately located around the end surface of the permanent magnet 36 therefore as the tone wheel 29 rotates all of the south poles or all of the north poles are simultaneously faces to all of the notches 33 all the way around also when identical poles are faces to all of the notches 33 all of the other poles are faced to the rotating magnetic tongue portions 42 located between the adjacent notches 33 first as shown in fig1 when all of the north poles located on the axially outer end face of the permanent magnet 36 are faced to all of the notches 33 the magnetic resistance between each north pole and the tone wheel 29 becomes large and it becomes difficult for.