The soft X-ray excess in the spectra of active galactic nuclei is characterized by
similar electron temperatures of 0.1-0.3 keV and similar photon indices around 2.2-3. It
remains a puzzle why both values are not sensitive to the black hole mass nor accretion
rate. Supposing that the scattering-dominated surface layer of an accretion disk can act
as a warm corona, we construct a vertically one-zone model to understand what determines
its temperature. By solving the equations of (1) the condition for the effective optical
depth, (2) the energy balance, and (3) dominance of the Compton cooling over the
bound-free cooling, we could reproduce the basic observational features of the soft
excess, provided that anomalous heating takes place in the warm corona. The similar
temperatures can be understood, since both of the anomalous heating and Compton cooling
rates are proportional to the dissipation rate of the accretion energy, while similar
photon indices are a natural consequence of the fact that observed photons are finally
emitted from the layer of Compton y ~ 1. The warm corona solutions only exist at smaller
radii, indicating the structure of a warm corona inside and a hot corona outside. The
soft excess is not observed in black hole binaries, since disk temperatures are too high
for the Compton scattering to work as cooling. The derived temperatures are somewhat
underestimation, however. This may indicate a necessity of multi-zone corona
structure. The stability of the warm corona and its consequences are briefly discussed.