We have investigated the electronic and optical properties of inorganic halide perovskites Cs2TiX6 (X = Br, I) using first-principles calculations based on the density functional theory, comparing them with those of CH3NH3PbX3 (X = Br, I). In particular, the absorption coefficients are calculated using the four-component spinors obtained as the solutions of the Dirac-Kohn-Sham equations. Our results show that Cs2TiX6 possess larger absorption coefficients than CH3NH3PbX3 in the main part of the solar spectrum. Furthermore, we calculate the nuclear quadrupole coupling constant C-Q of Br-79 and I-127 for Cs2TiX6 prior to experiments and evaluate the reliability of our calculated C-Q carefully. The reliability of the calculations is confirmed by comparing the calculated and experimental C-Q for the isostructural material K2PtBr6 as well as by examining those for MAPbX(3) and CsSnI3.