The effects of the addition of a metal ion in a Ni supported Ga2O3 photocatalyst on the photocatalytic overall splitting of H2O were investigated. The addition of Ca, Cr, Zn, Sr, Ba and Ta ions were effective in improving the photocatalytic activity. Particularly, the addition of the Zn ion improved the photocatalytic activity remarkably. The states of the photocatalyst after the addition of Zn ion are discussed.