The decay behavior of ${\text{CaAlSiN}}_3:{\text{Eu}}^{2+}$ was monitored as a function of the detection wavelength on the ${\text{Eu}}^{2+}$ emission band for various ${\text{Eu}}^{2+}$ concentrations from 0.0005 to 0.05. The decay at the higher-energy side was faster and less linear than the lower-energy side decay for all ${\text{Eu}}^{2+}$ concentrations. This was due to an inhomogeneous broadening induced by the random distribution of Al and Si ions at a crystallographic site in the ${\text{CaAlSiN}}_3$ structure and to the successive site-to-site energy transfer. This phenomenon caused a redshift of the ${\text{Eu}}^{2+}$ emission band as the ${\text{Eu}}^{2+}$ concentration increased.