The mechanism of ionic conduction in Ca-doped lanthanum oxychloride (LaOCl) was investigated using first-principles calculations based on density functional theory. The calculations of the point defect formation energies suggest that Cl- ion vacancies and substituted Ca2+ ions at La sites were dominant point defects. Although the migration energy of an O2- ion is 0.95 eV, the migration energy of a Cl- ion was calculated to be 0.44 eV, which is consistent with the reported experimental value. These results imply that the main carrier in Ca-doped LaOCl is Cl- ions and ionic conduction occurs by a Cl- ion vacancy mechanism.