  The environments of supernova explosion are often inhomogeneous and there may
be jumps in their density structure. We have developed a semi-analytic model
under the thin-shell approximation for supernova remnants that evolve crossing
a density jump in the ambient medium. The generic evolutionary relations are
presented for the blast wave after impacting on a cavity wall, which may be
produced by the energetic stellar wind from the supernova progenitor. The
relations can also be extended to the case that the blast waves break out from
a dense cloud if different density contrast is used. This model is applied to
N132D, a well-known cavity-born supernova remnant whose evolution has not yet
been quantitively estimated in a cavity scenario due to lack of model formulae,
and self-consistent physical parameters are obtained.
