To provide a rotation detecting device with a function of transmitting a rotational speed detection signal wireless and enable the rotation detecting device to be assembled compact in size without requiring any anti-rusting treatment to a multi-pole magnet assembly and a magnetic ring assembly, the rotation detecting device includes an electric power generator (11) for generating an electric power by a relative rotation between a non-rotatable member (1) and a rotatable member (7). The electric power generator (11) includes a multi-pole magnet assembly (13) and a magnetic ring assembly (12). A wireless transmission device (14) for transmitting the rotational speed detection signal outputted by the electric power generator (11) is employed. A sealing member (18) is also employed for sealing a working space delimited between the non-rotatable member (1) and the rotatable member (7), and the multi-pole magnet assembly (13) and the magnetic ring assembly (12) of the electric power generator (11) confront with each other at a location inwardly of the sealing member (18). The sealing member (18) is mounted on the magnetic ring assembly (12) or the wireless transmission device (14) of an annular configuration.