To provide a rotation detecting system of a type having a high detecting resolution, in which the rotational position can be detected accurately, in which a sufficient gap can be secured between the sensor and the rotating body and which is effective to simplify assemblage and processing to thereby reduce the manufacturing cost, the rotation detecting system is provided with a multiplying section 4 for multiplying the pulses, generated by the sensor 3, by a multiplication factor to form multiplied pulses Pb and a speed detecting section 5 for detecting an period average speed of the encoder 71 during the interval, in which the latest N pieces of the multiplied pulses Pb have been generated where N represents the multiplication factors by which the multiplied pulses Pb have been multiplied.