Abstract: Recent observations have found that massive galaxies with suppressed star formation activity exist even in the high redshift universe. On the other hand, it is poorly understood why they get quenched at such a high redshift and where they live in the large-scale structure. In this talk, I will introduce our two recent works on quiescent galaxies at z>2 in the COSMOS field. First, I report discovering an overdense structure of quiescent galaxies at z=2.77, which are spectroscopically confirmed by Keck/MOSFIRE observation. The four confirmed quiescent members imply that this structure is more than 68 times denser than the general field. This structure is likely a protocluster that will evolve into a Coma-like cluster at z=0. Next, we report X-ray (Chandra) and radio (VLA) image stacking analysis of quiescent galaxies to z~5. We detected the average stacked signal of quiescent galaxies in X-ray and radio. Their luminosity could not be explained without AGNs. Moreover, the AGN luminosity of quiescent galaxies is higher than those of star-forming galaxies with the same stellar mass. This could imply that the AGN activity may be related to quenching.