  We study dark soliton solutions of a multi-component Gross--Pitaevskii
equation for hyperfine spin F=1 spinor Bose--Einstein condensate. The
interactions are supposed to be inter-atomic repulsive and anti-ferromagnetic
ones of equal magnitude. The solutions are obtained from those of an integrable
$2\times 2$ matrix nonlinear Schr\"{o}dinger equation with nonvanishing
boundary conditions. We investigate the one-soliton and two-soliton solutions
in detail. One-soliton is classified into two kinds. The ferromagnetic state
has wavefunctions of domain-wall shape and its total spin is nonzero. The polar
state provides a hole soliton and its total spin is zero. These two states are
selected by choosing the type of the boundary conditions. In two-soliton
collisions, we observe the spin-mixing or spin-transfer. It is found that, as
"magnetic" carriers, solitons in the ferromagnetic state are operative for the
spin-mixing while those in the polar are passive.
