An experimental proof of self-reversal of thermo-remanent magnetization of igneous rocks is obtained. By cooling in a weak magnetic field, some igneous rocks and the ferromagnetic minerals separated from the mother rocks can have the remanent magnetization whose direction is opposite to that of the magnetic field during cooling. The processes of development and decay of the reverse thermo-remanent magnetization by cooling and heating respectively were directly observed (Figs. 11-19) and the characteristics of its appearance were examined. A few possible mechanisms of appearance of this peculiar phenomenon are discussed with reference to Néel's theory.