  The ground state magnetization process of the S=1 antiferromagnetic chain
with the easy-axis single-ion anisotropy described by negative $D$ is
investigated. It is numerically found that a phase transition between two
different gapless phases occurs at an intermediate magnetic field between the
starting and saturation points of the magnetization for $-1.49<D<-0.35$. The
transition is similar to the spin flopping, but it is second-order and not
accompanied with any significant anomalous behaviors in the magnetization
curve. We also present the phase diagrams in the m-D and H-D planes which
reveal a possible re-entrant transition.
