Abstract: A photocatalytic substance whose photocatalytic efficiency is greatly strengthened by making an improvement over titanium dioxide on which micron-size fine metal particles are supported, a base material which holds this photocatalytic substance, and manufacturing methods therefor. By reducing the mean particle diameter of metal particles from the size of fine metal particles on the micron scale to the size of ultra-fine metal particles on the nano-scale, it was succeeded that the photocatalytic efficiency of photocatalytic substances such as titanium dioxide, etc. was greatly strengthened. In particular, the feature is that the particle diameter of the ultra-fine metal particles used is set in a range which allows the conspicuous manifestation of a quantum size effect, and the mean particle diameter is in the range of 1 to 10 nm.