We have measured, for the first time, the light-intensity dependence in photocatalytic decomposition of water over K4Nb6O17. In higher light-intensity region, the rate of reaction is proportional to the square root of the light intensity. In lower light-intensity region, on the contrary, the rate is almost linearly proportional to the light intensity. We propose a reaction model whose main path of the reaction loss is the recombination of the charges generated under band-gap radiation. This model describes the light-intensity dependence of this reaction well.