A speed detecting device , comprising: a rotating body including a permanent magnet , the rotating body being spaced above a principal surface of a moving body , the moving body being a conductor configured to move or rotate, the permanent magnet being configured to rotate about a predetermined rotation axis by a Lorentz force produced in accordance with a moving or rotating direction of the moving body ; a detector configured to detect a rotating speed of the rotating body ; a storage unit configured to store a correlation between a moving or rotating speed of the moving body and a rotating speed of the rotating body based on a magnetic coupling degree between the moving body and the rotating body ; a moving speed estimating unit configured to estimate a moving or rotating speed of the moving body from the rotating speed of the rotating body detected by the detector , based on the correlation stored into the storage unit, a magnetic coupling degree measuring unit configured to measure a degree of magnetic coupling, based on a gap between the rotating body and the moving body and a degree of overlapping between the respective opposed surfaces of the rotating body and the moving body ; and a correlation detecting unit configured to detect a correlation between the moving or rotating speed of the moving body and the rotating speed of the rotating body and storing the correlation into the storage unit, for each of a plurality of degrees of magnetic coupling between the rotating body and the moving body , wherein the moving speed estimating unit is configured to estimate the moving or rotating speed of the moving body , based on, among a plurality of correlations detected by the correlation detecting unit , a correlation corresponding to the degree of magnetic coupling measured by the magnetic coupling degree measuring unit . a rotating body including a permanent magnet , the rotating body being spaced above a principal surface of a moving body , the moving body being a conductor configured to move or rotate, the permanent magnet being configured to rotate about a predetermined rotation axis by a Lorentz force produced in accordance with a moving or rotating direction of the moving body ; a detector configured to detect a rotating speed of the rotating body ; a storage unit configured to store a correlation between a moving or rotating speed of the moving body and a rotating speed of the rotating body based on a magnetic coupling degree between the moving body and the rotating body ; a moving speed estimating unit configured to estimate a moving or rotating speed of the moving body from the rotating speed of the rotating body detected by the detector , based on the correlation stored into the storage unit, a magnetic coupling degree measuring unit configured to measure a degree of magnetic coupling, based on a gap between the rotating body and the moving body and a degree of overlapping between the respective opposed surfaces of the rotating body and the moving body ; and a correlation detecting unit configured to detect a correlation between the moving or rotating speed of the moving body and the rotating speed of the rotating body and storing the correlation into the storage unit, for each of a plurality of degrees of magnetic coupling between the rotating body and the moving body , wherein the moving speed estimating unit is configured to estimate the moving or rotating speed of the moving body , based on, among a plurality of correlations detected by the correlation detecting unit , a correlation corresponding to the degree of magnetic coupling measured by the magnetic coupling degree measuring unit . <EOS>