  Near-infrared spectra of 15 high-redshift radio galaxies (HzRGs) located at
$2<z<2.6$ were obtained by the OH Airglow Suppressor spectrograph mounted on
the Subaru telescope. The UV-optical line ratio diagnostic diagrams indicate
that half of the observed HzRGs have extended emission-line regions with low
metal abundance, photoionized by a flat-continuum active galactic nucleus such
as a quasar. We also found two probable correlations between radio and
rest-optical parameters: (1) HzRGs with massive hosts tend to have a redder
rest-optical continuum, and (2) HzRGs with smaller radio sizes also show a
redder optical continuum. On the basis of the correlations, the nature of HzRGs
at $2<z<2.6$ is discussed.
