  We report the Fermi Large Area Telescope (Fermi-LAT) detection of the
$\gamma$-ray emission toward the massive star forming region of Carina Nebula
Complex (CNC). Using the latest source catalog and diffuse background models,
we found that the GeV $\gamma$-ray emission in this region can be resolved into
three different components. The GeV $\gamma$-ray emission from the central
point source is considered to originate from the Eta Carina ($\eta$ Car). We
further found the diffuse GeV $\gamma$-ray emission around the CNC which can be
modelled by two Gaussian disks with radii of 0.4\deg (region A) and 0.75\deg
(region B), respectively. The GeV $\gamma$-ray emission from both the regions A
and B have good spatial consistency with the derived molecular gas in
projection on the sky. The GeV $\gamma$-ray emission of region A reveals a
characteristic spectral shape of the pion-decay process, which indicates that
the $\gamma$-rays are produced by the interactions of hadronic cosmic rays with
ambient gas. The $\gamma$-rays spectrum of region B has a hard photon index of
2.12 $\pm$ 0.02, which is similar to other young massive star clusters. We
argue that the diffuse GeV $\gamma$-ray emission in region A and region B
likely originate from the interaction of accelerated protons in clusters with
the ambient gas.
