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.