The cobaloxime complex with the β-cyanoethyl group and isonicotinic acid as axial ligands forms host-guest complexes with several kinds of host molecules. Whe the crystals of the host-guest complexes were irradiated with visible light, the β-cyanoethyl group of the guest molecule was changed to the α-cyanoethyl group. The rate of the photoisomerization was controlled by the shape of the host molecule. Designing the host structures, the isotructural host-guest crystals were prepared and the controlled reaction rates were obtained.