  The importance of the interstellar magnetic field is studied in relation to
the evolutions of superbubbles with a three-dimensional (3D) numerical
magnetohydrodynamical (MHD) simulation. A superbubble is a large supernova
remnant driven by sequential supernova explosions in an OB association. Its
evolution is affected by the density stratification in the galactic disk. After
the size reaches 2--3 times the density scale-height, the superbubble expands
preferentially in the $z$-direction. Finally it can punch out the gas disk
(blow-out). On the other hand, the magnetic field running parallel to the
galactic disk has an effect to prevent from expanding in the direction
perpendicular to the field. The density stratification and the magnetic fields
have completely opposite effects on the evolution of the superbubble. We
present results of 3D MHD simulation in which both effects are included. As a
result, it is concluded that when the magnetic field has a much larger
scale-height than the density, even for a model that the bubble would blow out
from the disk if the magnetic field were absent, the magnetic field with the
strength of 5 $\mu$G can confine the bubble in $|z| \alt 300$ pc for $\simeq$20
Myr (confinement). In a model that the field strength decreases in the halo in
proportion to $B \propto \rho^{1/2}$, the superbubble eventually blows out like
a model of B=0 even if the magnetic field in the mid-plane is as strong as
$B=5\mu$G.
