Solid solutions between LaTaON2 and SrTiO3 were successfully synthesized. In this solid solution, the valence band maxima (VBM) shifted to positive sides as the fraction of SrTiO3 increased, resulting in the appearance of activity for O2 evolution even without any cocatalysts. Despite widening of the band gap (Eg) by only 0.15 eV, La0.5Sr0.5Ta0.5Ti0.5O2N gained 0.5 V larger driving force for water oxidation, which corresponds to the potential difference between VBM and potential for water oxidation, than LaTaON2. It suggested that the contribution of Ti 3d to the conduction band suppressed the widening of Eg. This is an interesting feature in the present LaTaON2-SrTiO3 solid solution. The bare La0.5Sr0.5Ta0.5Ti0.5O2N exhibited the activity for O2 evolution 9 times higher than LaTaON2 modified with a CoOx cocatalyst. It has been found that the driving force for water oxidation of La0.5Sr0.5Ta0.5Ti0.5O2N was 0.8 V, and was larger than those in other perovskite-type oxynitrides.