The effect of the preparation method on Ce3+ ion doping into CaS crystals is examined. When CaS:Ce is prepared in a H2S gas flow without any flux, at least two different kinds of Ce3+ centers are introduced into the CaS host. On the other hand, when alkali compounds are used as the flux during preparation, only one Ce3+ center is observed. This makes for differences in luminescence, excitation spectra, and temperature dependence of luminance between the two preparation methods. It was also found that CaS:Ce, prepared with codopant ions, has shallower trap states with a lower concentration than it does without a codopant.