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Near-infrared spectra of four QSOs located at $z>6$ are obtained with theOH-airglow suppressor mounted on the Subaru telescope. The FeII/MgIIemission-line ratios of these QSOs are examined by the same fitting algorithmas in our previous study of $z<5.3$ QSOs. The fitting results show that two outof 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 fittingalgorithm to more than 10,000 SDSS QSOs and found two trends in thedistribution of FeII/MgII against redshift: (1) the upper envelope of theFeII/MgII distribution at $z>3$ shows a probable declination toward highredshift, and (2) the median distribution settles into lower ratios at $z\sim1.5$ with small scatter compared to the other redshift. We discuss an Fe/Mgabundance evolution of QSOs with a substantial contribution from the diversenature of the broad-line regions in high-redshift QSOs.Comment: 12 pages, 2 figures. Accepted for publication in ApJ (10 October  2004, v614

Feii/MgiiEmission‐Line Ratios of QSOs. II.z> 6 Objects