A composite of titanium(IV) oxide (TiO 2 ) particles (core) and nanoporous silica (shell) was prepared by successive coating of a carbon layer and an octadecyl-functionalized silica layer on TiO 2 , followed by heat treatment to remove the organic components. Transmission electron microscope (TEM) observation and nitrogen (N 2 ) sorption analyses showed that the composite has a unique rattle-type structure, i.e., TiO 2 particles were encapsulated in the hollow silica shell having well-developed porosity. When the photocatalytic activity for gas phase decomposition of acetone over the composite was compared with that over naked TiO 2 without the lateral silica shell, the activity over the composite tended to become higher than that over naked TiO 2 as the initial amount of acetone in the system was reduced. The enhancement of decomposition rate under a diluted condition was due to condensation of acetone on the lateral silica shell, which resulted in enhancement of the collision rate between the substrate and the surface of the TiO 2 core.