We present results from our surveys to find rare AGN populations. We combined the XMM-Newton serendipitous source catalogue with wide-area multiwavelength surverys including Subaru Hyper Suprime-Cam, AKARI, and WISE, and developed efficient ways to find various rare AGN populations. The selection methods and properties of AGNs with a low-mass central black hole, AGNs deeply buried in obscuring matter, infrared bright AGNs, X-ray bright optically faint AGNs will be summarized.