  The evolution of supernova remnants (SNRs) is studied, with particular
attention to the effect of magnetic fields with axisymmetric two-dimensional
magnetohydrodynamical simulations. The evolution of magnetic SNRs is the same
as non-magnetic ones in the adiabatic Sedov stage. After a thin shell is
formed, the shell is driven by the pressure of the hot interior gas (bubble).
Evolution in the pressure-driven snow-plow phase is much affected by the
magnetic field. The shell sweeping the magnetic field lines thickens owing to
the magnetic pressure force. After $5\times 10^5{\rm yr}$ - $2\times 10^6{\rm
yr}$, the inner boundary of the thick shell begins to contract. This compresses
the hot bubble radially and maintains its thermal pressure. Thus, the bubble
forms a prolate spheroidal shape and becomes thinner and thinner, since it
expands in a direction parallel to the magnetic field for $B_0 \gtrsim 3\mu
{\rm G}$. Finally, the bubble contracts. The porosity of the hot low-density
gas in ISM is reduced, taking the effect of the magnetic field into account.
