A SO3 decomposition is common chemical reaction employed by thermochemical water-splitting cycles for hydrogen. This reaction proceeds endothermically in catalytic bed; in heat utilization system of the VHTR, needed heat is provided from high temperature helium gas by heat exchange. To specify sizes of this reactor, simultaneous analyses is necessary, in which the SO3 kinetics and heat exchange between the process gas and the helium are joined. This paper reports a kinetics formulation of a regression produced by Pt catalyst experiment; by computing heat exchange type reactor with the kinetics, temperature distributions and gas compositions are discussed to investigate required heat transfer area for achieving nearly chemical equilibrium.