A speed detecting device, comprising: an excitation coil for generating a magnetic flux corresponding to an alternating current; and a plurality of detection coils, each of the plurality of detection coils adapted to generate an induced voltage corresponding to a magnetic flux produced by an eddy current generated on a conductive relative moving body formed of a magnetic material, the eddy current being generated so as to correspond to a moving speed of the relative moving body moving through the magnetic flux generated by the excitation coil, characterized by further comprising: a gap estimation portion adapted to estimate, based on a current (i inj) flowing through the excitation coil and a voltage across the excitation coil, a gap (g) between the relative moving body and the excitation coil; and a speed estimation portion adapted to estimate, based on the gap (g) estimated by the gap estimation portion and the induced voltage generated in each of the plurality of detection coils, the moving speed of the relative moving body, wherein the gap estimation portion is configured to estimate the gap (g) based on a comparison between an inductance (L ic) or an impedance of the excitation coil calculated based on the current (i inj) and the voltage, and a known reference inductance (L ic,nom) or a known reference impedance of the excitation coil corresponding to a preset reference value (g nom) of the gap (g). an excitation coil for generating a magnetic flux corresponding to an alternating current; and a plurality of detection coils, each of the plurality of detection coils adapted to generate an induced voltage corresponding to a magnetic flux produced by an eddy current generated on a conductive relative moving body formed of a magnetic material, the eddy current being generated so as to correspond to a moving speed of the relative moving body moving through the magnetic flux generated by the excitation coil, characterized by further comprising: a gap estimation portion adapted to estimate, based on a current (i inj) flowing through the excitation coil and a voltage across the excitation coil, a gap (g) between the relative moving body and the excitation coil; and a speed estimation portion adapted to estimate, based on the gap (g) estimated by the gap estimation portion and the induced voltage generated in each of the plurality of detection coils, the moving speed of the relative moving body, wherein the gap estimation portion is configured to estimate the gap (g) based on a comparison between an inductance (L ic) or an impedance of the excitation coil calculated based on the current (i inj) and the voltage, and a known reference inductance (L ic,nom) or a known reference impedance of the excitation coil corresponding to a preset reference value (g nom) of the gap (g). a gap estimation portion adapted to estimate, based on a current (i inj) flowing through the excitation coil and a voltage across the excitation coil, a gap (g) between the relative moving body and the excitation coil; and a speed estimation portion adapted to estimate, based on the gap (g) estimated by the gap estimation portion and the induced voltage generated in each of the plurality of detection coils, the moving speed of the relative moving body, wherein the gap estimation portion is configured to estimate the gap (g) based on a comparison between an inductance (L ic) or an impedance of the excitation coil calculated based on the current (i inj) and the voltage, and a known reference inductance (L ic,nom) or a known reference impedance of the excitation coil corresponding to a preset reference value (g nom) of the gap (g).