To provide a sensor, which has practical sensitivity and high azimuth accuracy and which can be miniaturized and of which cost can be reduced, by focusing the earth magnetism with a core made of ferromagnetic body, and converting it to a high magnetic flux density, and detecting this conversion with a magneto resistive(MR) elements arranged in a gap between cores.

Ferromagnetic body cores K1-K4 made of permalloy or the like are combined so as to be formed into the square shape with gaps G1-G4 at the predetermined width and provided on a board 1. A sensor chip 2, in which each MR element M1-M4 is arranged, is provided in the upper part of the cores K1-K4 formed with the gaps. Temperature characteristic correcting MR elements M5, M6 are provided at a central part of the core K3, and MR elements M7, M8 are provided at a central part of the core K4 in series corresponding to each other so as to prevent the influence of the bias magnetic field to be generated from exciting coils C1, C2 provided in the cores K1, K2. Earth magnetism at a high output can be thereby detected at a high accuracy.