  We calculate the dynamical spin-structure factor of the $S=1$ Ising spin
chain with negative single-ion anisotropy in magnetic fields using the infinite
time-evolving-block-decimation algorithm. We show that when a transverse
magnetic field is applied, both the $\Delta S=2$ excitation continuum and
one-magnon mode appear in the low-lying excitation. When a longitudinal
magnetic field is further applied, the excitation continuum changes into
quantized excitation spectra. The quantized $\Delta S=2$ excitation spectra
originate from the confinement of two domain walls, each of which carries
$\Delta S=1$. The quantized excitation energies are explained by the negative
zeros in the Airy function.
