  To investigate the chemical abundance of broad-line region clouds in quasars
at high redshifts, we performed near-infrared spectroscopy of six luminous
quasars at z ~ 2.7 with the WINERED spectrograph mounted on the New Technology
Telescope (NTT) at the La Silla Observatory, Chile. The measured Fe II/Mg II
flux ratios nearly matched with the published data for 0.7 < z < 1.6,
suggesting that there is no evolution over a long period of cosmic time, which
is consistent with previous studies. To derive the chemical abundances from the
measured equivalent widths (EWs), their dependence on nonabundance factors must
be corrected. In our previous paper, we proposed a method to derive the [Mg/Fe]
abundance ratio and the [Fe/H] abundance by correcting the dependence of EW(Mg
II) and EW(Fe II) on the Eddington ratio. To the best of our knowledge, that
was the first report to discuss the star-formation history through a direct
comparison with chemical evolution models. In the present study, we further
investigated the dependence of EWs on luminosity, which is known as the Baldwin
effect (BEff). Additional correction for the BEff significantly affects the
derived chemical abundances for the six luminous quasars at z ~ 2.7, and the
resultant abundances agree well with the prediction of chemical evolution
models. Given that most distant quasars found thus far are biased toward
luminous ones, the correction of the measured EWs for the BEff is crucial for
extending the chemical evolution study to higher redshifts.
