Sulfur-Iodine (SI) cycle is one of the most promising thermochemical water splitting cycles to produce hydrogen using high-temperature heat transferred from a VHTR (very high temperature gas-cooled reactor). It was originally proposed by General Atomics Co. in 1970s, which was composed of the following three steps: (1) Bunsen reaction: x I2 + SO2 + 2 H2O = 2 HIx + H2SO4, (2) HIx decomposition: 2 HI = H2 + I2, (3) H2SO4 decomposition: H2SO4 = H2O + SO2 + 1/2 O2. Decomposition of sulfuric acid to SO2, O2 and H2O is the step to require the highest temperature heat from a VHTR in the whole SI cycle. This reaction consists of two series reactions: 1) decomposition of sulfuric acid to SO3 and H2O at around or above 350^oC with or without catalysts, and 2) catalytic decomposition of SO3 to SO2 and O2 at 750^oC or higher temperature. He, a cooling gas of the VHTR pressurized over 40 bars, transfers the high-temperature heat to the reactor. In order to lessen the pressure difference at the reactor wall, the reactions need to be conducted under a pressure as high as the wall can endure even though the equilibrium conversions to SO3 and H2O, and SO2 and O2 unfavorably reduce according to the Le Chatelier's principle. Since the acidic reactants and products either in liquid phase or in gas phase are highly corrosive, only a few materials may be applied to this severe reaction condition of high temperature and pressure. Silicon carbide (SiC) is known a good candidate which is resistant to sulfuric acid and SO3 at high temperature, but it possesses ceramic-like properties so that several problems such as flexible connection of parts and high pressure sealing are still remaining. Pt is another material resistant to the acidic reactant and products at high temperature. Pt is also reported catalytically active for decomposition reactions of both sulfuric acid and SO3 even though its long-term stability at high temperature over 850^oC is still questionable. In addition, Pt cannot be solely used for the high-pressure reactor because of its softness. In this study, a Pt-lined Hastelloy-C tubular reactor was prepared and the sulfuric acid decomposition was tested with and without additional catalysts at pressurized condition over 10 bars. The Pt-lined tubular reactor of 19 mm ID and 2440 mm length was placed in a heater with six independently working heating zones. A 10 m coiled tube of Inconel 600 in another heater is placed in front of the Pt-lined tube in order to evaporate and heat the reactant up to 600^oC. The decomposition reactions were conducted at 800^oC to 900^oC and at 1 bar to over 10 bars with 95 wt% H2SO4. The surface of Pt lining showed excellent catalytic activity over a summed-up period of 150 h even though its overall surface area is superficially not as large as the supported Pt catalysts commercially available. The extent of dissociation reaction to SO2 and O2 was evaluated referring to the variation of temperature, pressure and the reactant residence time. Additional charge of 20g commercial Pt/alumina (2 wt% Pt) catalyst pellets did not remarkably enhance the yield of SO2 and O2, indicating that the heat transfer area was the reaction determining factor rather than the amount of catalytic species in this case. This preliminary result has proposed possibility of adopting Pt-lined metallic tubes as reactor parts for high-temperature high-pressure decomposition reaction of sulfuric acid.