  The Tibet air shower array, which has an effective area of 37,000 square
meters and is located at 4300 m in altitude, has been observing air showers
induced by cosmic rays with energies above a few TeV. We are planning to add a
large muon detector array to it for the purpose of increasing its sensitivity
to cosmic gamma rays in the 100 TeV (10 - 1000 TeV) energy region by
discriminating them from cosmic-ray hadrons. We report on the possibility of
detection of gamma rays in the 100 TeV energy region in our field of view,
based on the improved sensitivity of our air shower array deduced from the full
Monte Carlo simulation.
