Coke combustion inside the iron ore sintering process is a major source of NOx in the steel industry. All the associated combustion reactions can be categorized as packed-bed combustion. Therefore, detailed analysis of NOx emission inside the packed bed is necessary to suppress the emission of environmentally hazardous NOx. In this study, the time-course profile of the conversion rate of fuel-N in coke to NOx was determined during packed-bed combustion. The conversion rate was calculated at each reaction time by analyzing the results of in-situ gas measurement inside the iron ore sintering reactor during the combustion. Both the emission of NO and the conversion rate were found to be higher in the lower-temperature region in the combustion layer, corresponding to the beginning of coke combustion. This result proved that it is necessary to reduce NOx emission at the beginning of coke combustion inside the iron ore sintering process to reduce NOx emission from the sintering process.