A body-insertable device including a resonance circuit for emitting a resonance magnetic field by being induced by a drive magnetic field received from the outside. An external device for deriving position information of the resonance circuit by using a detection signal of detected resonance magnetic field. The external device includes a drive coil driving unit for outputting a drive signal having a predetermined frequency, a drive coil for receiving the output drive signal and generating the drive magnetic field, and a drive magnetic field generation control unit for calculating an amplitude value of the drive signal by using a current amplitude value of the drive signal, detecting a degree of stability indicating whether a sharp change occurs in the drive magnetic field based on the calculated amplitude value, and controlling the deriving of a position information based on the detected degree of stability in the drive magnetic field.