An external device (200) disposed on the outside of a detection space (K) in a position detecting magnetic guidance system (1) includes a signal generating unit (221) that outputs a drive signal with a resonance frequency F0； a drive coil driving unit (222) that amplifies the current of the drive signal generated by the signal generating unit (221)； a drive coil (224) for generating a drive magnetic field in the detection space (K) according to the drive signal input from the drive coil driving unit (222)； a control unit (201) (drive magnetic field generation controller 201A: see FIG. 6 ) that detects a degree of stability of the drive magnetic field generated by the drive coil (224) and controls the signal generating unit (221) based on the detected stability.
