Inhomogeneity is the most fundamental concept which characterizes the states of supercritical fluids, and density fluctuation is a measure of the inhomogeneity in the distribution of molecules. To investigate the substance dependence of the inhomogeneity, small-angle X-ray scattering experiments and thermodynamic calculations have been carried out for supercritical CO2 and CF3H and the contour maps and their ridges have been drawn on the phase diagrams. The contour maps and the ridges for both samples are similar and the magnitudes of the density fluctuation are almost the same at the same reduced thermodynamic states. The ridges connect smoothly with the rectilinear diameters. The physical meaning of a ridge is stated in terms of the effective volume occupied by molecules. It is concluded that the ridge is the boundary which separates the supercritical region into two a more liquid-like region and a more gas-like region.