  We present the results of our near-infrared (NIR) spectroscopic observations
of a rich cluster candidate around a radio galaxy at $z=1.52$ (4C65.22) with
Subaru/MOIRCS and LBT/LUCI. We observed 71 galaxies mostly on the star-forming
main sequence selected by our previous broad-band (photo-$z$) and narrow-band
H$\alpha$ imaging observation in this cluster environment. We successfully
confirmed the redshifts of 39 galaxies, and conclude that this is a
gravitationally bound, real cluster at $z=1.517$. Our spectroscopic data also
suggest a hint of large-scale filaments or sheet-like three-dimensional
structures crossing at the highest-density cluster core. By stacking the
spectra to derive their average interstellar medium (ISM) gas-phase metallicity
based on the [NII]/H$\alpha$ emission line flux ratio, we find that the
mass-metallicity relation (MZR) in the 4C65.22 cluster environment is
consistent with that of H$\alpha$-selected field galaxies at similar redshifts.
Our results suggest that the environmental impacts on the MZR is small at high
redshifts, but a larger sample of high-$z$ clusters and their member galaxies
is still required to fully address the effect of environment as well as its
cluster-cluster variation.
