  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 have a plan to add a
large muon detector array to it for the purpose of increasing its sensitivity
to cosmic gamma rays in the 100 TeV energy region by discriminating them from
cosmic-ray hadrons. We have deduced the attainable sensitivity of the muon
detector array using our Monte Carlo simulation. We report here on the detailed
procedure of our Monte Carlo simulation.
