The present invention relates to a method for the decomposition of the bromate ions contained in a liquid. This method includes the successive step of the irradiation with light contact with the liquid and the photocatalyst and, having not lower the photocatalyst than the band gap energy of said photocatalyst, to produce a photocatalytic reaction by him decompose bromate ions. The invention also relates to an apparatus for the decomposition of the bromate ions contained in a liquid. The apparatus comprises a first chamber for causing a photocatalytic reaction decompose bromate ions therein; Photocatalyst in contact with the liquid in the first chamber; And a light source for irradiating said photocatalyst with light rays is not lower than the photocatalyst having a band gap energy of the photocatalyst in the case of contact with the liquid to cause a photocatalytic reaction in the first chamber; includes. Therefore, it is possible to effectively and reliably the bromate ion decomposition. The photocatalyst may include at least one metal oxide having a lower electronegativity than titanium and titanium. In this case, it is possible to omit the adjustment of the pH of the liquid before and after the photocatalytic reaction.