We present the results of our systematic search for optically elusive, but intrinsically luminous buried AGNs in >100 nearby (z < 0.3) luminous infrared galaxies with L(IR) > 1011 L⊙, classified optically as non-Seyferts. To disentangle AGNs and stars, we have performed (1) infrared 2.5-35 μ low-resolution (R ~ 100) spectroscopy using Subaru, AKARI, and Spitzer, to estimate the strengths of PAH (polycyclic aromatic hydrocarbon) emission and dust absorption features, (2) high-spatial-resolution infrared 20 micron imaging observations using Subaru and Gemini, to constrain the emission surface brightnesses of energy sources, and (3) millimeter interferometric measurements of molecular gas flux ratios, which reflect the physical and chemical effects from AGNs and stars. Overall, all methods provided consistent pictures. We found that the energetic importance of buried AGNs is relatively higher in galaxies with higher infrared luminosities (where more stars will be formed), suggesting that AGN-starburst connections are luminosity dependent. Our results might be related to the AGN feedback scenario as the possible origin of the galaxy down-sizing phenomenon.