The photoreaction of xanthone in an alcoholic solution flowing through a column reactor filled with the mesoporous silica MCM-41 was performed. It was found that the product yields depend greatly on both the pore size of the filler and the nature of the solvent. The product yields of this reaction, which involves a pair of radicals as an intermediate, were dependent on the magnetic field as well. These results were interpreted with a model wherein the lifetime of the radical pair was elongated by several hundred times or more in the mesopore, or nanotube, compared with that in the bulk solution. This unusual phenomenon was attributed to the collective movement of the solvent molecules in the nanotube, which would thereby inhibit the translational diffusion of the solute molecules.