  We first present a self-consistent dynamical model in which $\omega$ Cen is
formed from an ancient nucleated dwarf galaxy merging with the first generation
of the Galactic thin disc in a retrograde manner with respect to the Galactic
rotation. Our numerical simulations demonstrate that during merging between the
Galaxy and the $\omega$ Cen's host dwarf with $M_{\rm B}$ $\simeq$ -14 mag and
its nucleus mass of $10^7$ $M_{\odot}$, the outer stellar envelope of the dwarf
is nearly completely stripped whereas the central nucleus can survive from the
tidal stripping because of its compactness. The developed naked nucleus has a
very bound retrograde orbit around the young Galactic disc, as observed for
$\omega$ Cen, with its apocenter and pericenter distances of $\sim$ 8 kpc and
$\sim$ 1 kpc, respectively. The Galactic tidal force can induce radial inflow
of gas to the dwarf's center and consequently triggers moderately strong
nuclear starbursts in a repetitive manner. This result implies that efficient
nuclear chemical enrichment resulting from the later starbursts can be closely
associated with the origin of the observed relatively young and metal-rich
stars in $\omega$ Cen. Dynamical heating by the $\omega$ Cen's host can
transform the young thin disc into the thick disc during merging.
