We built a large (approximately 4,000 m**2) water Cherenkov-
type muon detector array under the existing Tibet air shower array at
4,300 m above sea level, to observe 10-1000 TeV gamma rays from
cosmic-ray accelerators in our Galaxy with wide field of view
at very low background level. A gamma-ray induced air shower has
significantly less muons compared with a cosmic-ray induced one.
Therefore, we can effectively discriminate between primary gamma rays
and cosmic-ray background events by means of counting number of
muons in an air shower event by the muon detector array. We make a
status report on the experiment.