A cast piece surface temperature measuring device includes: a magnetic field exciting device which applies an AC magnetic field substantially perpendicular to a surface of a cast piece; a magnetic field detecting device which detects the AC magnetic field to detect a magnetic flux varied in response to a surface temperature of the cast piece; and a surface temperature deriving device which derives the surface temperature of the cast piece based on an induced electromotive force obtained by detecting the AC magnetic field by the magnetic field detecting device and a predetermined relation data. The magnetic field exciting device includes a solenoidal excitation coil, the magnetic field detecting device includes a solenoidal detection coil interposed between the surface of the cast piece and the excitation coil, and the relation data indicates a correspondence relationship between the surface temperature of the cast piece and the induced electromotive force in a temperature range including a predetermined Curie point.