  The diffuse Galactic $\gamma$-ray emission, mainly produced via interactions
between cosmic rays and the interstellar medium and/or radiation field, is a
very important probe of the distribution, propagation, and interaction of
cosmic rays in the Milky Way. In this work we report the measurements of
diffuse $\gamma$-rays from the Galactic plane between 10 TeV and 1 PeV
energies, with the square kilometer array of the Large High Altitude Air Shower
Observatory (LHAASO). Diffuse emissions from the inner
($15^{\circ}<l<125^{\circ}$, $|b|<5^{\circ}$) and outer
($125^{\circ}<l<235^{\circ}$, $|b|<5^{\circ}$) Galactic plane are detected with
$29.1\sigma$ and $12.7\sigma$ significance, respectively. The outer Galactic
plane diffuse emission is detected for the first time in the very- to
ultra-high-energy domain ($E>10$~TeV). The energy spectrum in the inner Galaxy
regions can be described by a power-law function with an index of
$-2.99\pm0.04$, which is different from the curved spectrum as expected from
hadronic interactions between locally measured cosmic rays and the
line-of-sight integrated gas content. Furthermore, the measured flux is higher
by a factor of $\sim3$ than the prediction. A similar spectrum with an index of
$-2.99\pm0.07$ is found in the outer Galaxy region, and the absolute flux for
$10\lesssim E\lesssim60$ TeV is again higher than the prediction for hadronic
cosmic ray interactions. The latitude distributions of the diffuse emission are
consistent with the gas distribution, while the longitude distributions show
clear deviation from the gas distribution. The LHAASO measurements imply that
either additional emission sources exist or cosmic ray intensities have spatial
variations.

High Energy Astrophysical Phenomena