  We have developed a Monte Carlo event generator for non-resonant diphoton
($\gamma\gamma$) production at hadron collisions in the framework of GR@PPA,
which consistently includes processes having additional one jet radiation. The
possible double count problem in the generation of radiative processes is
avoided by using the LLL subtraction method that we have applied to the
weak-boson production processes. The subtraction method has been extended to
the final-state QED divergence that appears in the $qg \rightarrow \gamma\gamma
+ q$ process. Because a parton shower (PS) which regularizes the subtracted QED
divergence is still under development, we tried to use PYTHIA for the
generation of the fragmentation events to restore the subtracted components.
The simulation employing the "old" PS of PYTHIA shows a reasonable matching
with the GR@PPA events, and the combined event sample shows a result in
reasonable agreement with ResBos. We found that the contribution from $qg
\rightarrow \gamma\gamma + q$ is significant in the LHC condition. This event
generator must be useful for the background studies in low-mass Higgs boson
searches at LHC.
