PURPOSE: To obtain a detection signal of a target substance irrespective of geomagnetism etc., by a method wherein a low-frequency part corresponding to an angular velocity of turning is cut by a low-cut filter from a magnetic field signal obtained by a magnetic field sensor and the part is used as a signal showing the presence absence of a target magnetic substance.

CONSTITUTION: A detecting apparatus of a magnetic substance is borne on an airplane so that a pickup coil 2 is directed in the advancing direction, and the plane is made to go round. A signal component obtained from turning and a signal component obtained from a submarine, a target magnetic substance, are mixed in a magnetic field signal Bx in the direction X which is obtained by a magnetic field sensor 1. From this signal Bx is low-frequency part is cut by a low-cut filter LCF 5. Since a cutoff frequency of the LCF 5 is determined by a voltage Vω corresponding to an angular velocity ω of turning, which is an output of a differentiator 10, the low-frequency part cut from the signal Bx turns to be a signal component which corresponds to the angular velocity of turning and varies slowly relatively.