We present the rest-frame optical spectra of high redshift QSOs taken with AKARI, a Japanese IR space telescope with Korea and ESA participation. High redshift QSOs hold an important key to the understanding of the growth of supermassive black-holes and their role in the galaxy formation. However, our understanding of high redshift QSOs has been hampered by the lack of the rest-frame optical spectra where exist the popular key spectral diagnostics to derive QSO properties such as masses of supermassive black-holes. Using the AKARI's unique spectroscopic capability at 2 - 5 micron, we observed 14 QSOs at 4.5 < z < 6.5, revealing for the first time, the optical emission lines such as Balmer lines of QSOs at z > 4.5. In this talk, we show the basic properties of high redshift QSOs derived from the rest-frame optical spectra, and discuss how such properties compare with QSOs at lower redshifts.
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