Apparatus for detecting rotation of a bicycle part

An apparatus for detecting rotation of a bicycle part comprises a casing member 23 comprising a generally annular body 67 structured to be mounted to the bicycle part so that the annular body 67 is incapable of rotating relative to the bicycle part as the bicycle part rotates in opposite directions around a rotational axis X. The annular body 67 includes a plurality of circumferentially disposed exposed magnet mounting portions 67a that are concentric with respect to the rotational axis X, and a cover member 68 is detachably mounted to the annular body 67 for blocking the plurality of magnet mounting portions 67a.