  Diffuse $\gamma$-ray emission is the most prominent observable signature of
celestial cosmic-ray interactions at high energies. While already being
investigated at GeV energies over several decades, assessments of diffuse
$\gamma$-ray emission at TeV energies remain sparse. After completion of the
systematic survey of the inner Galaxy, the H.E.S.S. experiment is in a prime
position to observe large-scale diffuse emission at TeV energies. Data of the
H.E.S.S. Galactic Plane Survey are investigated in regions off known
$\gamma$-ray sources. Corresponding $\gamma$-ray flux measurements were made
over an extensive grid of celestial locations. Longitudinal and latitudinal
profiles of the observed $\gamma$-ray fluxes show characteristic excess
emission not attributable to known $\gamma$-ray sources. For the first time
large-scale $\gamma$-ray emission along the Galactic Plane using imaging
atmospheric Cherenkov telescopes has been observed. While the background
subtraction technique limits the ability to recover modest variation on the
scale of the H.E.S.S. field of view or larger, which is characteristic of the
inverse Compton scatter-induced Galactic diffuse emission, contributions of
neutral pion decay as well as emission from unresolved $\gamma$-ray sources can
be recovered in the observed signal to a large fraction. Calculations show that
the minimum $\gamma$-ray emission from $\pi^0$-decay represents a significant
contribution to the total signal. This detection is interpreted as a mix of
diffuse Galactic $\gamma$-ray emission and unresolved sources.
