NOVELTY - The CPUs (31e,31g) calculate the heat energy released by the hydrogen and mixed gas individually using the output of the hydrogen and mixed gas sensors and density information stored in the memory (34a). The CPU (31h) calculates the heat energy released by the mixed gas of required density depending on the output of the CPUs (31e,31g).    USE - For determining the heat energy released during conversion of town gas to liquefied natural gas (LNG) in gas utility.    ADVANTAGE - The heat energy released in calculated correctly without using expensive gas chromatography.    DETAILED DESCRIPTION - A hydrogen density calculation CPU (31c) calculates hydrogen density in the mixed gas based on the output of the contact combustion type gas sensor (10) at low temperature and relationship of hydrogen density and high/low temperature stored in the memory (34b). A hydrogen sensor output CPU (31d) calculates hydrogen sensor output depending on the sensor output at high temperature, calculated hydrogen density and mixed gas density except hydrogen. The hydrogen exothermic energy calculation CPU (31e) calculate heat energy released by hydrogen gas depending on the hydrogen sensor output and corrects the heat energy calculation information in the memory (34a) so as to correspond to the mixed gas density. The mixed gas energy release calculation CPU (31g) calculates heat energy released by the mixed gas depending on the mixed gas sensor output and the corrected information in the memory. The exothermic energy calculation CPU (31h) calculates the heat energy released by the mixed gas of required density from the output of the CPUs (31g,31h).    INDEPENDENT CLAIMS are also included for the following:    (1) exothermic energy calculation method; and    (2) exothermic energy calculation system.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of the exothermic energy calculation apparatus. (Drawing includes non-English language text).    contact combustion type gas sensor (10)    CPUs (31a-31j)    memories (34a,34b)