We have investigated the photocatalytic activity of individual porous TiO2 nanotubes by the single-molecule counting of center dot OH using a specific fluorescent probe. The time- and space-resolved observation of emissive fluorescein generated by the photocatalytic reaction clearly reveals the importance of the transport behavior of reagents through the porous structures on the photocatalytic activity and the existence of the spatial heterogeneity of reactive sites even in an isolated TiO2 nanotube. An experiment on a single nanotube provided information that was useful for elucidating the reaction mechanism of the heterogeneous (photo)catalyst and for designing advanced porous materials.