  Near-infrared spectra of four QSOs located at $z>6$ are obtained with the
OH-airglow suppressor mounted on the Subaru telescope. The FeII/MgII
emission-line ratios of these QSOs are examined by the same fitting algorithm
as in our previous study of $z<5.3$ QSOs. The fitting results show that two out
of the four $z>6$ QSOs have significant FeII emission in their rest-UV spectra,
while the other two have almost no FeII features. We also applied our fitting
algorithm to more than 10,000 SDSS QSOs and found two trends in the
distribution of FeII/MgII against redshift: (1) the upper envelope of the
FeII/MgII distribution at $z>3$ shows a probable declination toward high
redshift, and (2) the median distribution settles into lower ratios at $z\sim
1.5$ with small scatter compared to the other redshift. We discuss an Fe/Mg
abundance evolution of QSOs with a substantial contribution from the diverse
nature of the broad-line regions in high-redshift QSOs.
