Abstract: The reasons causing the spontaneous magnetization and demagnetization of ferromagnetic materials staying in static state in the geomagnetic field are explained in the present paper. The first cause is the magnetic ageing——the magnetism of ferromagnetic materials and the spatial arrangements of magnetic domains all lag behind the magnetizing field applied upon them. The second one is periodical changes of the geomagnetic field strength in magnitudes and directions with the passage of time. And the final cause is the ageing of metals——the increasing of mechanical stress in the materials versus time induces the change of its magnetization curve and metallographic constitutional changes as well as the appearance of the delayed cracking etc.