  Threshold current of domain wall motion under spin-polarized electric current
in ferromagnets is theoretically studied based on the equation of motion of a
wall in terms of collective coordinates. Effects of non-adiabaticity and a
so-called $\beta$-term in Landau-Lifshitz equation, which are described by the
same term in the equation of motion of a wall, are taken into account as well
as extrinsic pinning. It is demonstrated that there are four different regimes
characterized by different dependence of threshold on extrinsic pinning,
hard-axis magnetic anisotropy, non-adiabaticity and $\beta$.
