  We propose a mean-field approach to analyze many-body systems of fermions in
the gauge/gravity duality. We introduce a non-vanishing classical fermionic
field in the gravity dual, which we call the holographic mean field for
fermions. The holographic mean field takes account of the many-body dynamics of
the fermions in the bulk. The regularity condition of the holographic mean
field fixes the relationship between the chemical potential and the density
unambiguously. Our approach provides a new framework of gauge/gravity duality
for finite-density systems of baryons in the confinement phase.
