  We investigate the S=1 antiferromagnetic quantum spin chain with the exchange
and single-ion anisotropies in a magnetic field, using the numerical exact
diagonalization of finite-size clusters, the level spectroscopy analysis, and
the density matrix renormalization group (DMRG) method. It is found that a
translational symmetry broken magnetization plateau possibly appears at the
half of the saturation magnetization, when the anisotropies compete with each
other. The level spectroscopy analysis gives the phase diagram at half the
saturation magnetization. The DMRG calculation presents the magnetization
curves for some typical parameters and clarifies the spin structure in the
plateau phase.
