To provide a wall thinning detection sensor which can detect by a remote field method considerably correctly and accurately an abrasion and thinning of outer face of a ferromagnetic pipe, by enhancing excitation efficiency of an exciting coil and detection sensitivity to a magnetic field.

A wall thinning detection sensor 1 magnetizes a ferromagnetic pipe 100 with an a.c. by an exciting coil 2 arranged inside the pipe. A phase change of an a.c. magnetic field is detected by magnetic sensors 3 separated by an optional distance in the pipe. A thinning of the pipe is thus inspected. The exciting coil 2 is formed by winding a coil 23 around a magnetic core 21 coaxially set with the pipe 100. A disk 22 of magnetic material which has a smaller diameter than an inner diameter of the pipe 100 is arranged at one end or both ends of the magnetic core 21. The magnetic sensors 3 are amorphous magnetic sensors.