In this study, we investigated about the effect of the unsteady diffusion on the reaction rate of an immobilized catalyst in a batch type reaction system. Analyzing the diffusion and reaction under the unsteady state, we showed that true kinetic information can be obtained at the limiting condition of beta --> 0, where beta is the volume ratio of the solid to liquid. For the definite value of beta, we can derive an equation that expresses the relation between the apparent rate constant and beta. This knowledge is valuable for studying immobilized catalyst kinetics.