The photocatalytic degradation over TiO2 particles suspending in the gas-liquid dispersion was modeled by taking the decay of the illumination intensity in the gas-liquid-solid fluidized bed in terms of Lambert-Beer law into consideration. For the sake of experimental verification of the proposed model, the oxidative degradation of an acid dye (C.I. Acid Blue 40) as a model compound, was carried out using a coaxial double cylinder-type reactor (internal cylinder part: quartz glass cylinder installing black light type fluorescent tube as UV light source; annular part: gas-liquid-solid fluidized bed) newly manufactured here. The reactor is operated continuously with respect to the gas phase and batchwise with respect to the solid phase, whereas the liquid phase is recirculated. The degradation rate could be expressed apparently as first-order in the concentration of acid dye. The observed relationship between apparent degradation rate constant and solid (TiO2) loading could satisfactorily be interpreted in terms of the proposed model.