Magnetic field effect (MFE) on the photoinduced electron transfer reaction between benzophenone and starburst dendrimers are studied in water by laser flash photolysis. In the presence of a magnetic field of 0.6 T, the relative yields of benzophenone anion radical at a 4-7 mu s delay increase by about 158 in the case of 3-, 4-, and 5-generation dendrimers, whereas there is no appreciable MFE in the case of 1- and 2-generation dendrimers, 1,3-diaminopropane, and N,N,N',N'-tetraethyl-1,3-diaminopropane. The MFE is interpreted in terms of a radical pair mechanism. Present results indicate clearly that the dendrimers of higher generation act not only as an electron donor but also as a supercage in the photoreaction.