  We present new deep optical spectra of 9 high-z radio galaxies (HzRGs) at z >
2.7 obtained with FORS2 on VLT. These rest-frame ultraviolet spectra are used
to infer the metallicity of the narrow-line regions (NLRs) in order to
investigate the chemical evolution of galaxies in high-z universe. We focus
mainly on the CIV/HeII and CIII]/CIV flux ratios that are sensitive to gas
metallicity and ionization parameter. Although the NV emission has been widely
used to infer the gas metallicity, it is often too weak to be measured
accurately for NLRs. By combining our new spectra with data from the
literature, we examine the possible redshift evolution of the NLR metallicity
for 57 HzRGs at 1 < z < 4. Based on the comparison between the observed
emission-line flux ratios and the results of our photoionization model
calculations, we find no significant metallicity evolution in NLRs of HzRGs, up
to z ~ 4. Our results imply that massive galaxies had almost completed their
chemical evolution at much higher redshift (z > 5). Finally, although we detect
strong NV emission lines in 5 HzRGs at z > 2.7, we point out that high NV/HeII
ratios are not indicative of high metallicities but correspond to high
ionization parameters of gas clouds in NLRs.
