PURPOSE: To detect simply both a magnetic field and a magnetic field gradient, by a method wherein a magnetic field signal and a difference signal between this signal and a delayed magnetic field signal obtained by delaying the aforesaid signal by a prescribed time are used as signals for estimating the presence or absence of a target magnetic substance.

CONSTITUTION: A detecting apparatus of a magnetic substance is borne on an airplane and the plane is made to go round at a speed (v) in a prescribed direction. A magnetic field signal Bx is detected by a magnetic field sensor 1 composed of a pickup coil 2, SQUID 3 and a detecting circuit 4. Besides, a difference signal ΔBx of the magnetic field signals between positions being apart from each other by a distance in a straight line L=vT is obtained by a delay circuit 5, a delay time setting device 6 and a differential amplifier 10. The presence or absence of a submarine, a target magnetic substance, can be judged from the aforesaid difference signal ΔBx when it navigates in the shallow sea and from the afore-said magnetic field signal Bx when it navigates in the deep sea.