PURPOSE: To enable high-speed switching of large magnetic fields by reinforcing two high magnetic permeability materials parallel with the plane of a magneto-optical disk in a magnetic path respectively by metals and specifying the inductances of core windings.

CONSTITUTION: This electromagnet is constituted of the high magnetic permeability material 1 which is installed to face the cylindrical high magnetic permeability material 2 wound with the winding 23 with the magneto-optical disk 8 held in place therebetween and has a cylindrical hollow part 21 wound with the winding 5, the high magnetic permeability materials 3 and 4 which magnetically couple the high magnetic permeability materials 1 and 2, reinforcing metallic plates 6, 7 which reinforce the high magnetic permeability materials 1 and 3 and a circuit 10 for modulating the current for generating the magnetic field which passes the same current to the windings 5 and 23. Nonmagnetic materials such as aluminum, titanium, copper and zinc are used for the metal for reinforcement and the inductances of the core windings are specified to ≤20μH. The inductances of the core windings are decreased and the closed magnetic path is constituted in such a manner and, therefore, the high-speed switching of the large magnetic fields is enabled.