Mesoporous silica, MCM-41, was packed in a pyrex column and used as a flow reactor for a solution system. The reactant solution, containing xanthone and xanthene, was irradiated with UV light while flowing in the column reactor under a magnetic field. When MCM-41 with a pore diameter of 2.5 nm was employed, a large increase in the 'cage reaction' yield was observed in comparison with that for MCM-41, 3.9 nm. About 25% of this nanopore effect was canceled by the magnetic field of 500 mT.