Magnetic fields of active supermassive black holes have not been measured yet, although it is one of the most fundamental parameters in black hole physics such as for launching mechanism of relativistic jets and generation of hot coronae. In this talk, I will report our new measurements of magnetic fields in several tens Schwarzschild radii of central black holes. Utilizing ALMA and NuSTAR, we successfully detected coronal synchrotron emission. The estimated magnetic field strength is not strong enough to keep corona hot.