When two ferromagnetic layers are separated by an insulator thin layer, electrical resistance of the multilayer in the perpendicular direction to the film changes depending on the orientations of the magnetizations of ferromagnetic thin layers because of spin dependent electron tunneling between the two ferromagnetic layers. When the directions of the magnetizations of the two ferromagnetic electrodes are the same, the possibility of electron tunneling between the two ferromagnetic electrode through the insulator layer becomes larger, resulting in larger tunneling current. However, if the directions of the magentizations of the two ferromagnetic electrodes are opposite, the electron with opposite spin orientation with respect to the magnetization of the ferromagnetic electrode cannot be tunneled. Then the tunneling electron current become smaller compared to the case for the same directions of the magnetizations. This phenomenon is called tunneling magnetoresistance (TMR), because its value is reaching to 100% at room temperature.