We present the results of global three dimensional magnetohydrodynamic simulations of galactic gaseous disks. For the gravitational potential we adopt a model of our galaxy including dark matter given by Miyamoto et al. (1980). The initial gas distribution is that of an equilibrium torus in the gravitational potential. We assume initially weak toroidal magnetic fields. As the magnetorotational instability develops the torus becomes turbulent and changes itself into a disk. Magnetic energy increases and saturates when B ∼ 1μG. The magnetic field is maintained for more than 2.0 × 109 yrs. Although magnetic fields inside the disk are turbulent, global mean azimuthal field appears. The direction of the mean azimuthal field changes with the height from the equatorial plane. We also present the results of global MHD simulations under non-axisymmetric bar potential.