  We observed atmospheric gamma-rays around 10 GeV at balloon altitudes (15~25
km) and at a mountain (2770 m a.s.l). The observed results were compared with
Monte Carlo calculations to find that an interaction model (Lund Fritiof1.6)
used in an old neutrino flux calculation was not good enough for describing the
observed values. In stead, we found that two other nuclear interaction models,
Lund Fritiof7.02 and dpmjet3.03, gave much better agreement with the
observations. Our data will serve for examining nuclear interaction models and
for deriving a reliable absolute atmospheric neutrino flux in the GeV region.
