It has been suggested that Nitrogen oxide (NOx) in the unburned air-fuel mixture promotes spontaneous ignition, according to the results of the latest calculations and experiments based on the combustion reaction mechanism. If we consider the influence this has on internal combustion engines, NOx in residual gas is to affect knock characteristics. This paper describes how the authors elucidated how NOx affects knock in spark-ignition engines and calculated the level of spontaneous ignition of unburned gas taking into account the influence of compression brought by flame propagation. A study was also conducted into how NOx affects knock by operating a single-cylinder gas engine with Ar-O2 pseudo-air as the operating fluid. It was consequently discovered that there was a close correlation between knock and the level of NOx in the unburned air-fuel mixture. The major influence that NOx has on promoting the spontaneous ignition of the unburned air-fuel mixture was also verified by means of test results and a study of the process of how knock occurs. By running an engine with pseudo-air of Ar-O2 as the operating fluid, it was also found that an increase in combustion chamber temperature resulted in a large increase in combustion speed, and that this increased combustion speed had the added effect on reducing unburned air-fuel mixture in the exhaust gas.