  A structure model for black holes is proposed by mean field approximation of
gravity. The model, which consists of a charged singularity at the center and
quantum fluctuation of fields around the singularity, is similar to the atomic
structure. The model naturally quantizes the black hole. Especially we find the
minimum black hole, whose structure is similar to the hydrogen atom and whose
Schwarzschild radius becomes about 1.1287 of the Planck length.
