A rotation detector that can be assembled into a hub of a wheel without breakage of an object to be detected. The rotation detector has a housing (1) having an insertion section (1a) in which an axle (S) is inserted and that is placed between a hub (10a) of a front wheel (10) and a front fork, a magnet (3a) placed in the housing (1) and generating a magnetic field, and a magnetism detection element (3) provided in the housing (1) so as to be able to detect a change in the magnetic field and detecting the change in the magnetic field involved in the rotation of the object (5) to be detected. The object (5) to be detected is formed of a different magnetic material from the hub (10a) and has a portion (5a) to be detected that rotates in synchronous with the hub (10a).