  The magnetization curve of the $(S,s)=(1,1/2)$ ferrimagnetic alternating spin
chain with the single-ion anisotropy $D$ is investigated with the numerical
exact diagonalization of finite clusters and size-scaling analyses. The system
has a plateau at 1/3 of the saturation moment, which corresponds to the
spontaneous magnetization for D=0. Varying $D$ in the 1/3-magnetized ground
state under the external field along the axis of $D$, a quantum phase
transition is revealed to occur at the critical value $D/J=1.114 \pm 0.001$
where the plateau vanishes. Except for the critical point, the plateau is
always opening, but the mechanism is different between $D<D_c$ and $D>D_c$. The
change of mechanisms is an evidence to clarify that the plateau originates from
the quantization of magnetization.
