  Using high-resolution N-body/SPH simulations with $2\times 10^6$ particles,
we investigate the evolution of stellar and gaseous galactic cores during the
hierarchical formation of a spiral galaxy. We find that the galactic core ($r <
300 $ pc) coevolves with the host galaxy. The average mass ratio between the
baryonic core and the halo is nearly constant, $\sim $ 0.04 from $z \sim 10$ to
$z \sim 2$. However, there are several `rapid-growing phases' during the
evolution, in which the rate of mass accretion to the central sub-kpc region is
ten times higher ($\sim 1 M_\odot$ yr pc$^{-1}$) than the average accretion
rate. The rapid growth of the inner core is associated with the major merger
events with a time-delay. We also find that the spin-axis of the gas core
frequently changes. As a result, the angular momentum vector of the central
part of the galaxy is independent of the rotation of the outer part. Our
results suggest that if a constant fraction of the baryonic mass in the central
several 100 pc of a galaxy is converted into a massive black hole, the black
hole mass should correlate with the total mass of the galaxies.
