The photocatalytic activity for water decomposition of RuO2-dispersed ZnGa2O4 with an octahedrally coordinated Ga3+ ion was studied in order to examine the photocatalytic properties of p-block metal oxides with d(10) configuration. ZnGa2O4 was impregnated with Ru-3(CO)(12) or Ru(C5H7O2)(3) in THF and oxidized to convert them to RuO2. For RuO2-loaded ZnGa2O4, hydrogen and oxygen were stably produced from the initial stage,of reaction under Hg-Xe lamp light irradiation. The activity dependence on the calcination temperature of ZnGa2O4, the amount of RuO2 loaded, and the oxidation temperature of RuO2 showed that the dispersion of small RuO2 particles leads to high photocatalytic activity. The electronic structure of ZnGa2O4 was calculated by a plane-wave-based density function theory (DFT). The valence bands were mainly composed of the O 2p orbitals, whereas the conduction bands were formed by hybridzation of Ga 4s4p and Zn 4s4p orbitals. Large dispersion observed in the conduction band indicated the large mobility of photoexcited electrons, which was considered to be responsible for high photocatalytic performance. The feature of electronic structures relating to photocatalysis was discussed in comparison with those of transition metal oxides.