  A new calculation of the Galactic diffuse gamma-ray spectrum from the decay
of secondary particles produced by interactions of cosmic-ray protons with
interstellar matter is presented. The calculation utilizes the modern Monte
Carlo event generators, Hadrin, Fritiof and Pythia, which simulate high-energy
proton-proton collisions and are widely used in studies of nuclear and particle
physics, in addition to scaling calculation. This study is motivated by the
result on the Galactic diffuse gamma-ray flux observed by the EGRET detector on
the Compton Gamma-ray Observatory, which indicates an excess above about 1 GeV
of the observed intensity compared with a model prediction. The prediction is
based on cosmic-ray interactions with interstellar matter, in which secondary
pion productions are treated by a simple model. With the improved interaction
model used here, however, the diffuse gamma-ray flux agrees rather well with
previous calculations within uncertainties, which mainly come from the
unobservable demodulated cosmic-ray spectrum in interstellar space. As a
possible solution to the excess flux, flatter spectra of cosmic-ray protons
have been tested and we found that the power-law spectrum with an index of
about $-(2.4\sim2.5)$ gives a better fit to the EGRET data, though the spectrum
is not explained completely.
