Effects of metal ions on catalytic activities of photocatalysts for decomposition of fluoroborate were studied. Unmodified photocatalysts exhibited poor activities. TiO₂(P-25) whose activity was the best showed maximum removal ratio of BF₄^- at pH near 4, which could be explained as followed: Due to the isoelectric point of TiO₂ at about 4 of pH, the surface charge of the TiO₂ became positive at slightly lower pH value than that of the point, and that of BF₄^- was still so negative that the adsorption which enabled the photodecomposition to take place could be facilitated. Cr/TiO₂ and Fe/TiO₂ photocatalysts exhibited higher activities than photocatalysts doped by the other metal ions used. In particular, 0.5 wt% Cr/TiO₂ and 1.0 wt% Fe/TiO₂ showed maximum activities whose appearances could be explained by the decrease of the thickness of the space charge layer as the content of dopant increased.