  Velocity distribution of dark matter is assumed to be isotropic in most
cases, however, anisotropy is suggested in some simulations. Directional direct
detection of dark matter is a hopeful way to discriminate the anisotropy of
dark matter velocity distribution. We simulate the dark matter and target
scattering in the directional direct detection, and investigate conditions
required to discriminate the anisotropy. If dark matter mass is known,
$O(10^3)-O(10^4)$ events are required for the discrimination if the dark matter
mass is known by other experiments. We also study the case that the dark matter
mass is not known, and in analysis using both the recoil energy and the
scattering angle data, both the dark matter mass and the anisotropy can be
restricted much better than the analysis only with either of them.
