Abstract: A system for setting an analysis condition for a thermal analysis of a fluid inside an apparatus, wherein a gas flow-in temperature in the apparatus, a gas flow-in rate, and an apparatus outer wall temperature are set as critical conditions, the interior of the apparatus being under a high pressure atmosphere and heated by a heater, an analysis mesh shape, a pressure inside the apparatus and other values, and a heater power, are set as initial values, and a simulation is carried out so that an optimum operating condition is obtained, wherein the relationship between heater power by which a heater monitoring temperature is maintained at a constant value and a gas flow rate at the entrance while changing the apparatus internal pressure is obtained by a trial experiment, and the simulation is carried out by changing the gas flow rate at the entrance in accordance with the heater power and the gas flow rate at the entrance set as the initial value.