A divalent copper salt and/or trivalent iron salt is supported on a surface of a metal ion-doped titanium oxide obtained by doping titanium oxide with metal ions to give a metal ion-doped titanium oxide with a valence band potential of 3 V or more (vs. SHE, pH = 0) and a bandgap of 3 V or less between the valence band and an energy level of electrons excited from the valence band (including conduction band minimum potential and isolated potential). The metal ion-doped titanium oxide can be made to exhibit strong oxidative decomposition activity when irradiated with visible light based on the fact the divalent copper salt or trivalent iron salt functions as a catalyst for multi-electron reduction of oxygen.
