The authors inserted an optical fiber into the combustion chamber of a DI engine and investigated the diesel flame spectra. The engine, attached with a water spray device in the intake pipe, was operated under various ignition timings, and the relations were investigated among the intensity of CN band spectra, NOx concentration in the diesel exhaust and the flame temperature. As the result, a certain correlation was found between CN band spectra and NOx concentration. From this correlation, they confirmed that the NOx in the diesel exhaust contains not only the generation by “the Zeldvich mechainism”, but also “the Prompt NOx”. They also confirmed that to reduce “the Prompt NOx”, it is not effective to retard the ignition, but effective to spray water into the combustion chamber in high load operations.