We investigated the photocatalytic decomposition of 3000 ppmv of acetaldehyde in an air stream using 0.1 wt% of platinum supported on TiO2/SiO2 catalyst and TiO2/SiO2 catalyst for comparison. When the distance between the catalyst surface and the light source was set at 8 mm, the conversions of acetaldehyde were almost the same between TiO2/SiO2 and Pt-TiO2/SiO2 catalysts in the reaction temperature of about 370K. On the other hand, when the distance was changed to 3 mm (the reaction temperature of about 420K), the acetaldehyde conversion on the TiO2/SiO2 catalyst became quite different from that on the Pt-TiO2/SiO2 catalyst. The conversion reached nearly 100%. This result is considered to be due to accumulation of soft coke and coke precursors produced from acetaldehyde and then their thermocatalytic combustion. It is concluded that the Pt-TiO2/SiO2 catalyst IS n effective and multifunctional catalyst, because TiO2 decomposes organic compounds, porous SiO2 support adsorbs the by-products and does not release them to the outside of the reactor, and platinum easily bums by-products and coke precursors. We also investigated the preparation method to obtain a highly active multi-functional catalyst.