The measured pressure distributions of the blunt-nosed cone test model in Free-piston shock tunnel HIEST placed in JAXA Kakuda space center were compared with calculated results. At high total enthalpy conditions, the calculated pressure distribution was higher than the experimental one. As the cause for this discrepancy, first we focused on the chemical nonequilibrium model in the supersonic expanding region. Flowfield calculation with changes of chemical reaction rates of Park's model was conducted and the effect on pressure distribution was examined. Accelerating the dissociation reaction rate of O2 had most effect on the pressure coefficients. A good agreements with experimental values was obtained. Second, the effect of chemical composition in freestream was also investigated. The more undisociated O2 in freestream resulted in the more reduction of pressure. This implies that it is necessary to identify chemical compositions in freestream and evaluation of aerodynamic characteristics.