A practically realizable semiconductor magnetic body having a flat-band structure is disclosed. The semiconductor magnetic body is formed by semiconductor quantum dots arranged on lattice points such that electrons can transfer between neighboring quantum dots and the electron energy band contains a flat-band structure, where each quantum dot is a structure in which electrons are confined inside a region which is surrounded by high energy potential regions, and the flat-band structure is a band structure in which energy dispersion of electrons has hardly any wave number dependency.