  The nuclear gas kinematics probed by water vapor maser emission has shown
that two nearby active galaxies, NGC 1068 and NGC 4258, have a supermassive
object in their nuclei and their masses are nearly comparable; a few 10^7 solar
masses. Nevertheless, the activity of the central engine of NGC 1068 is more
powerful by two orders of magnitude than that of NGC 4258. Since it is
generally considered that the huge luminosities of active galactic nuclei are
attributed to the mass accretion onto a supermassive black hole, the above
observational results suggest that the accretion rate in NGC 1068 is much
higher than that in NGC 4258. Comparing the kinematical properties of the
accreting molecular clouds between NGC 1068 and NGC 4258, we find possible
evidence for dynamical gas accretion in NGC 1068, which may be responsible for
the more powerful central engine in this galaxy.
