  We investigate the $S=2$ antiferromagnetic quantum spin chain with the
exchange and single-ion anisotropies in a magnetic field, using the numerical
exact diagonalization of finite-size clusters and the level spectroscopy
analysis. It is found that a magnetization plateau possibly appears at a half
of the saturation magnetization for some suitable anisotropy parameters. The
level spectroscopy analysis indicates that the 1/2 magnetization plateau is
formed by two different mechanisms, depending on the anisotropy parameters. The
phase diagram of the 1/2 plateau states and some typical magnetization curves
are also presented. In addition the biquadratic interaction is revealed to
enhance the plateau induced by the Haldane mechanism.
