  A linear stability analysis has been performed onto a self-gravitating
magnetized gas disk bounded by external pressure. The resulting dispersion
relation is fully explained by three kinds of instability: a Parker-type
instability driven by self-gravity, usual Jeans gravitational instability and
convection. In the direction parallel to the magnetic fields, the magnetic
tension completely suppresses the convection. If the adiabatic index $\gamma$
is less than a certain critical value, the perturbations trigger the Parker as
well as the Jeans instability in the disk. Consequently, the growth rate curve
has two maxima: one at small wavenumber due to a combination of the Parker and
Jeans instabilities, and the other at somewhat larger wavenumber mostly due to
the Parker instability. In the horizontal direction perpendicular to the
fields, the convection makes the growth rate increase monotonically upto a
limiting value as the perturbation wavenumber gets large. However, at small
wavenumbers, the Jeans instability becomes effective and develops a peak in the
growth rate curve. Depending on the system parameters, the maximum growth rate
of the convection may or may not be higher than the peak due to the
Jeans-Parker instability. Therefore, a cooperative action of the Jeans and
Parker instabilities can have chances to over-ride the convection and may
develop large scale structures of cylindrical shape in non-linear stage. In
thick disks the cylinder is expected to align its axis perpendicular to the
field, while in thin ones parallel to it.
