To provide a sensor, which can improve the working accuracy and which can be miniaturized and of which the cost can be reduced and which has a high detecting sensitivity, by providing magneto resistive(MR) elements on a gap so as to nearly cross the magnetic field direction in a magnetic gap of a core made of the ferromagnetic body.

Ferromagnetic body cores K1, K2 made of ferrite or the like are made to abut on each other in each joining surface S so as to be formed into the square shape. The cores K1, K2 are formed with gaps G1, G2 and G3, G4 made of the glass material, and each RM element M1-M4 is formed on the gap. Exciting coils C1, C2 made of Cu are wound around the cores K1, K2 at a central part thereof, and the bias magnetic field is applied to the MR elements M1-M4. The bias magnetic field respectively displaced at 180 degree is applied to the X direction detecting MR elements M1, M2 and the Y direction detecting MR element M3, R4. Resistance value of the MR elements M1-M4 is changed by the intensity of the magnetic field, and they have the excellent ability for sensing the weak magnetic field.