  An event generator for diphoton ($\gamma\gamma$) production in hadron
collisions that includes associated jet production up to two jets has been
developed using a subtraction method based on the LLL subtraction. The parton
shower (PS) simulation to restore the subtracted divergent components involves
both QED and QCD radiation, and QED radiation at very small $Q^{2}$ are
simulated by referring to a fragmentation function (FF). The PS/FF simulation
has the ability to enforce the radiation of a given number of energetic
photons. The generated events can be fed to PYTHIA to obtain particle
(hadron)-level event information, which enables us to perform realistic
simulations of photon isolation and hadron-jet reconstruction. The simulated
events, in which the loop-mediated $gg \rightarrow \gamma\gamma$ process is
involved, reasonably reproduce the diphoton kinematics measured at the LHC.
Using the developed simulation, we found that the 2-jet processes significantly
contribute to diphoton production. A large 2-jet contribution can be considered
as a common feature in electroweak-boson production in hadron collisions
although the reason is yet to be understood. Discussion concerning the
treatment of the underlying events in photon isolation is necessary for future
higher precision measurements.
