Abstract: A magnetic field detecting device, particularly for detecting the earth's magnetic field, includes one or more amorphous or microcrystalline magnetic fibres having two bistable magnetization states and surrounded by one single measuring coil (103). The measuring coil is supplied with a periodically changing control signal (101a) which is superposed with a detected signal (103a) contingent on the sensed magnetic field. The detected signal is caused to control a flip-flop (122) which influences the working direction of an integrator (101), the output signal of which forms a performance signal in addition to forming said control signal. The performance signal (101c, 106a) is processed in a microprocessor (107), to which a control signal (122b) is also delivered from the flip-flop, and is able to restart the circuit when a control signal fails to appear.