We present the results of our systematic search for optically elusive,
but intrinsically luminous buried AGNs in nearby (z < 0.3) ultraluminous
infrared galaxies (ULIRGs) classified optically as non-Seyferts. To
disentangle AGNs and stars, we have performed (1) infrared 2.8-4.1
micron low-resolution (R ~ 100) spectroscopy using Subaru IRCS to
estimate the strength of PAH (polycyclic aromatic hydrocarbon) emission
and dust absorption features, and (2) 20 micron imaging observations
of selected ULIRGs using Subaru COMICS and Gemini TReCS, to constrain
the emission surface brightnesses of energy sources. Overall, both
methods provide consistent results about the importance of buried AGNs. 
By combining with AKARI IRC 2.5--5 micron and Spitzer IRS 5-35 micron
low-resolution spectroscopy, we found that luminous buried AGNs become 
relative more important with increasing galaxy infrared luminosity
(where more stars will be formed). Our results may support the AGN
feedback scenario as the origin of galaxy down-sizing phenomenon.