In a speed detecting apparatus for a rotating body, a correction coefficient is used to correct a detecting error in each of pulse signal periods representing speed data of the rotating body. The correction coefficient is updated so as to cancel an aberration of the corresponding pulse signal period caused by a nonstandard factor of the rotating body. Specifically, an average value of the pulse signal periods corresponding to one rotation of the rotating body is first derived, then a value indicative of a deviation between corresponding one of the pulse signal periods and the average value is derived, and further, an updating value is derived by adjusting a degree of influence of the deviation indicative value to the correction coefficient using a correction sensitivity coefficient. The correction coefficient is updated by adding the updating value to a last value of the correction coefficient.