  We present the production cross-section of gamma-rays based on data of p-p
collisions at LHC, revising the previous semi-empirical formula mainly for 1)
the inelastic cross-section in p-p collisions, $\sigma_{pp}(E_0)$, and 2) the
inclusive gamma-ray spectrum in the forward region, $\sigma_{pp \rightarrow
\gamma}(E_0, E_\gamma)$. We find that the previous cross-section gives a
significantly softer spectrum than found in the data of LHC. In this paper, we
focus our interest mainly upon the LHC forward (LHCf) experiment, giving
gamma-ray spectra in the very forward region with the pseudo-rapidity $\eta^*$
> 8.8 in the center of mass system (CMS), which have not been reported so far.
We also give the pseudo-rapidity distribution of charged hadrons with -3 <
$\eta^*$ < 3 obtained by ALICE and TOTEM experiments, both with LHC. We find
that the revised cross-section reproduces quite well the accelerator data over
the wide energy range from GeV to 30 PeV for projectile protons, corresponding
approximately to 100 MeV to 3 PeV for secondary gamma-rays. The production
cross-section of gamma-rays produced in the forward region is essential for the
study of gamma-ray astronomy, while not important are those produced in the
central region in CMS, and of much less importance in the backward. We discuss
also the average transverse momentum of gamma-rays, $\bar{p}_{t}$, and the
average inelasticity transferred to gamma-rays, $\bar{k}_\gamma^*$, obtaining
that the former increases very slowly with $\bar{p}_{t}$ = 100 - 220 MeV/c for
$E_0$ = 1 GeV - 26 PeV, and the latter is almost independent of $E_0$, with
$\bar{k}_\gamma^* \approx 1/6$, while we can not exclude the possibility of a
small increase of $\bar{k}_\gamma^*$.
