  Deep mid-infrared (10-20 $\mu$m) images with sub-arcsec resolution were
obtained for two Herbig Be stars, MWC 1080 and HD 259431, to probe their
immediate environments. Our goal is to understand the origin of the diffuse
nebulosities observed around these two very young objects. By analyzing our new
mid-IR images and comparing them to published data at other wavelengths, we
demonstrate that the well extended emission around MWC 1080 traces neither a
disk nor an envelope, but rather the surfaces of a cavity created by the
outflow from MWC 1080A, the primary star of the MWC 1080 system. In the N-band
images, the filamentary nebulosities trace the hourglass-shaped gas cavity wall
out to $\sim$0.15 pc. This scenario reconciles the properties of the MWC 1080
system revealed by a wide range of observations. Our finding confirms that the
environment around MWC 1080, where a small cluster is forming, is strongly
affected by the outflow from the central Herbig Be star. Similarities observed
between the two subjects of this study suggest that the filamentary emission
around HD 259431 may arise from a similar outflow cavity structure, too.
