To reconcile both of the miniaturization of an axial dimension and the improvement of the output of a sensor 27.

An encoder is formed into the shape of a single cylinder as a whole by directly connecting both axial ends of a first cylindrical part 17 capable of mounting a part to be detected 38 and a second cylindrical part 18 capable of fitting and fixing an inner ring 10, having the same diameter. The axial dimension of the whole encoder 16 can be reduced for the omission of a ring part and the like for forming a stepped part between the first and second cylindrical parts 17, 18, while ensuring the axial dimension of the part to be detected 38.