  We carried out global three-dimensional resistive magnetohydrodynamic
simulations of galactic gaseous disks to investigate how the galactic magnetic
fields are amplified and maintained. We adopt a steady axisymmetric
gravitational potential given by \cite*{Miy80}. As the initial condition, we
assume a warm ($T\sim10^{5} \mathrm{K}$) rotating gas torus centered at
$\varpi=10 \mathrm{kpc}$ threaded by weak azimuthal magnetic fields. Numerical
results indicate that in differentially rotating galactic gaseous disks,
magnetic fields are amplified due to magneto-rotational instability and
magnetic turbulence develops. After the amplification of magnetic energy
saturates, the disk stays in a quasi-steady state. The mean azimuthal magnetic
field increases with time and shows reversals with period of 1Gyr (2Gyr for a
full cycle). The amplitude of $B_{\phi}$ near the equatorial plane is
$B_{\phi}\sim1.5\mu \mathrm{G}$ at $\varpi=5 \mathrm{kpc}$. The magnetic fields
show large fluctuations whose standard deviation is comparable to the mean
field. The mean azimuthal magnetic field in the disk corona has direction
opposite to the mean magnetic field inside the disk. The mass accretion rate
driven by the Maxwell stress is $\sim10^{-3}M_{\odot}/ \mathrm{yr}$ at
$\varpi=2.5 \mathrm{kpc}$ when the mass of the initial torus is
$\sim5\times10^{8}M_{\odot}$.
