We present the results of our systematic infrared 2.5–35 μm low‐resolution spectroscopy of nearby ultraluminous infrared galaxies (ULIRGs) at z<0.3, using Subaru, AKARI, and Spitzer. We detected the signatures of optically‐elusive, but intrinsically luminous, buried AGNs in a significant fraction of ULIRGs classified optically as non‐Seyferts. We found that the energetic importance of buried AGNs depends on galaxy infrared luminosity, suggesting that the AGN‐starburst connections are luminosity dependent. It is suggested that buried AGNs are energetically more important in the progenitors of more massive galaxies, which might be related to the AGN feedback scenario for the galaxy downsizing phenomenon.