  Dark matter annihilation in galactic substructure would imprint
characteristic angular signatures on the all-sky map of the diffuse gamma-ray
background. We study the gamma-ray background anisotropy due to the subhalos
and discuss detectability at Fermi Gamma-ray Space Telescope. We derive
analytic formulae that enable to directly compute the angular power spectrum,
given parameters of subhalos. As our fiducial subhalo models, we adopt M^{-1.9}
mass spectrum, subhalos radial distribution suppressed towards the galactic
center, and luminosity profile of each subhalo dominated by its smooth
component. We find that, for multipole regime corresponding to \theta <~ 5 deg,
the angular power spectrum is dominated by a noise-like term, with suppression
due to internal structure of relevant subhalos. If the mass spectrum extends
down to Earth-mass scale, then the subhalos would be detected in the anisotropy
with Fermi at angular scales of ~10 deg, if their contribution to the gamma-ray
background is larger than ~20%. If the minimum mass is around 10^4 M_{sun}, on
the other hand, the relevant angular scale for detection is ~1 deg, and the
anisotropy detection requires that the subhalo contribution to the gamma-ray
background intensity is only ~4%. These can be achieved with a modest boost for
particle physics parameters. We also find that the anisotropy analysis could be
a more sensitive probe for the subhalos than individual detection. We also
study dependence on model parameters, where we reach the similar conclusions
for all the models investigated. The analytic approach should be very useful
when Fermi data are analyzed and the obtained angular power spectrum is
interpreted in terms of subhalo models.
