  In this Letter we show that the vacuum polarization of quantum fields in an
anti-de Sitter space naturally gives rise to a small but nonzero cosmological
constant in a brane world living in it. To explain the extremely small ratio of
mass density in the cosmological constant to the Planck mass density in our
universe (\approx 10^{-123}) as suggested by cosmological observations, all we
need is a four-dimensional brane world (our universe) living in a
five-dimensional anti-de Sitter space with a curvature radius r_0 \sim
10^{-3}cm and a fundamental Planck energy M_P \sim 10^9 GeV, and a scalar field
with a mass m \sim r_0^{-1}\sim 10^{-2}eV. Probing gravity down to a scale \sim
10^{-3}cm, which is attainable in the near future, will provide a test of the
model.
