We study the structure of a disk-corona system in pressure balance as a model for central engines of active galactic nuclei and Galactic black hole candidates. We assume that a fraction f of gravitational energy is dissipated in a corona, and the remaining fraction, 1 - f within a disk. The disk is heated not only by viscous processes (the α-model) but also by the irradiation from the corona. Cooling of the corona is due to bremsstrahlung and Comptonization of soft photons supplied by the disk. We find two distinct branches of steady state solutions for a given luminosity, depending on the main cooling source in the corona; one is bremsstrahlung-dominated (the b-branch), and the other is Comptonized-soft-photon dominated (the s-branch). The emission spectrum from the s-branch obeys a power law, while the emission from the b-branch is the superposition of bremsstrahlung from the corona and a blackbody from the disk. The s-branch was previously obtained by Haardt & Maraschi, but the newly found b-branch affects the whole structure of the solution.The structure of a disk-corona system is very sensitive to a parameter ζ = (1 -f)/α and the dissipation rate Qg of gravitational energy in a unit area. When we fix the radial distance from the central black hole, R, two solutions exist for any of Qg only if ζ is smaller than a critical value, ζcr (≫1). When ζ > ζcr, in contrast, there is a range of Qg where no steady state solution exists, because the cooling rate exceeds the heating rate. On the other hand, when the mass accretion of a disk, Mdotd, is given, there is an annular region where no steady state solution exists when ζ > ζRcr. Here the critical value ζRcr (≫1) depends on Mdotd.