When at least one of a distribution of gas composition and distribution of temperature inside a furnace is estimated by dividing the region inside the furnace into two-dimensional or three-dimensional cells, and calculating a gas flow rate, a gas reaction amount, a coal combustion rate, a radiant heat transfer rate in each of the cells, based on design data including furnace dimentions and operational data including a coal supply rate and an air supply rate, an air ratio of gas phase--gas composion table is referred to, thereby to simplify the gas reaction amount calculation and reduce drastically time required for the calculation.