  Strong soft X-ray emission called soft X-ray excess is often observed in
luminous active galactic nuclei (AGN). It has been suggested that the soft
X-rays are emitted from a warm ($T=10^6\sim10^7\ \rm{K}$) region that is
optically thick for the Thomson scattering (warm Comptonization region).
Motivated by the recent observations that soft X-ray excess appears in changing
look AGN (CLAGN) during the state transition from a dim state without broad
emission lines to a bright state with broad emission lines, we performed global
three-dimensional radiation magnetohydrodynamic simulations assuming that the
mass accretion rate increases and becomes around $10$\% of the Eddington
accretion rate. The simulation successfully reproduces a warm, Thomson-thick
region outside the hot radiatively inefficient accretion flow near the black
hole. The warm region is formed by efficient radiative cooling due to inverse
Compton scattering. The calculated luminosity $0.01L_{{\rm Edd}}-0.08L_{{\rm
Edd}}$ is consistent with the luminosity of CLAGN. We also found that the warm
Comptonization region is well described by the steady model of magnetized disks
supported by azimuthal magnetic fields. When the anti-parallel azimuthal
magnetic fields supporting the radiatively cooled region reconnect around the
equatorial plane of the disk, the temperature of the region becomes higher by
releasing the magnetic energy transported to the region.
