In order to develop a high temperature heat storage and temperature upgrading system using the CaO/CaCO3 reaction, decarbonation of CaCO3 has been carried out with thermogravimetry in the ranges of 1073-1193 K and 3-55 kPa, Calcium carbonate virtually did not decompose under the condition where the partial pressure of CO2, P, is higher than half of the equilibrium decomposition pressure of CaCO3, P-e, It implies that the present heat storage system has to be operated in pressure regions lower than half of P-e. Most decarbonation occurred during the chemical reaction-controlling period. A rate expression, employing a fractional pressure term, (P-e - P)/P-e, is proposed for the reaction-controlling period by assuming the presence of a reaction intermediate. It is found that the dependency of pressure on reaction rate is nonlinear, and the reaction is strongly influenced by the change in P.