  A new calculation of nuclear enhancement factor, used in estimation of
Galactic diffuse gamma-ray flux from proton-proton interaction in order to take
account of heavy nuclei included in cosmic-rays and interstellar matter, is
presented by use of a Monte Carlo simulator, DPMJET-3. A new value of 1.8-2.0
in the energy range of 6-1000 GeV/nucleon, slightly increasing with kinetic
energy of projectile cosmic rays, is about 20% larger than previous estimates.
