We study the dark matter content in early-type galaxies and investigate whether X-ray luminosities of early-type galaxies are determined by the surrounding gravitational potential. We derived gravitational mass profiles of 22 early-type galaxies observed with XMM-Newton and Chandra. Sixteen galaxies show constant or decreasing radial temperature profiles, and their X-ray luminosities are consistent with kinematical energy input from stellar mass loss. The temperature profiles of the other 6 galaxies increase with radius, and their X-ray luminosities are significantly higher. The integrated mass-to-light ratio of each galaxy is constant at that of stars within 0.5–1re, and increases with radius. The scatter of the central mass-to-light ratio of galaxies was less in K-band light. At 3re, the integrated mass-to-light ratios of galaxies with flat or decreasing temperature profiles are twice the value at 0.5re, where the stellar mass dominates, and at 6re, these increase to three times the value at 0.5re. This feature should reflect common dark and stellar mass distributions in early-type galaxies: Within 3re, the mass of dark matter is similar to the stellar mass, while within 6re, the former is larger than the latter by two-fold. In contrast, X-ray luminous galaxies have higher gravitational mass in the outer regions than X-ray faint galaxies. We describe these X-ray luminous galaxies as the central objects of large potential structures; the presence or absence of this potential is the main source of the large scatter in the X-ray luminosity.