  The Galactic diffuse emission (GDE) is formed when cosmic rays leave the
sources where they were accelerated, diffusively propagate in the Galactic
magnetic field, and interact with the interstellar medium and interstellar
radiation field. GDE in $\gamma$-ray (GDE-$\gamma$) has been observed up to
sub-PeV energies, though its origin may be explained by either cosmic-ray
nuclei or electrons. We show that the $\gamma$-rays accompanying the
high-energy neutrinos recently observed by the IceCube Observatory from the
Galactic plane have a flux that is consistent with the GDE-$\gamma$ observed by
the {\it Fermi}-LAT and Tibet AS$\gamma$ experiments around 1 TeV and 0.5 PeV,
respectively. The consistency suggests that the diffuse $\gamma$-ray emission
above $\sim$1TeV could be dominated by hadronuclear interactions, though
partial leptonic contribution cannot be excluded. Moreover, by comparing the
fluxes of the Galactic and extragalactic diffuse emission backgrounds, we find
that the neutrino luminosity of the Milky Way is one to two orders of magnitude
lower than the average of distant galaxies. This implies that our Galaxy has
not hosted the type of neutrino emitters that dominates the isotropic neutrino
background at least in the past few tens of kiloyears.
