  Gamma rays from HESS J1849$-$000, a middle-aged TeV pulsar wind nebula (PWN),
are observed by the Tibet air shower array and the muon detector array. The
detection significance of gamma rays reaches $4.0\, \sigma$ and $4.4\, \sigma$
levels above 25 TeV and 100 TeV, respectively, in units of Gaussian standard
deviation $\sigma$. The energy spectrum measured between $40\, {\rm TeV} < E <
320\, {\rm TeV}$ for the first time is described with a simple power-law
function of ${\rm d}N/{\rm d}E = (2.86 \pm 1.44) \times 10^{-16}(E/40\, {\rm
TeV})^{-2.24 \pm 0.41}\, {\rm TeV}^{-1}\, {\rm cm}^{-2}\, {\rm s}^{-1}$. The
gamma-ray energy spectrum from the sub-TeV ($E < 1\, {\rm TeV}$) to sub-PeV
($100\, {\rm TeV} < E < 1\, {\rm PeV}$) ranges including the results of
previous studies can be modeled with the leptonic scenario, inverse Compton
scattering by high-energy electrons accelerated by the PWN of PSR J1849$-$0001.
On the other hand, the gamma-ray energy spectrum can also be modeled with the
hadronic scenario in which gamma rays are generated from the decay of neutral
pions produced by collisions between accelerated cosmic-ray protons and the
ambient molecular cloud found in the gamma-ray emitting region. The cutoff
energy of cosmic-ray protons $E_{\rm p\, cut}$, cut is estimated at ${\rm
log}_{10}(E_{\rm p,\, cut}/{\rm TeV}) = 3.73^{+2.98}_{-0.66}$, suggesting that
protons are accelerated up to the PeV energy range. Our study thus proposes
that HESS J1849$-$000 should be further investigated as a new candidate for a
Galactic PeV cosmic-ray accelerator, PeVatron.
