CaB6 single crystals were synthesized by the Al flux method and the Fe-impurity concentration in the crystal was measured using the PIXE method. As a result, we found that the amount of the Fe impurity, included in a CaB6 powder sample used as a starting material, is drastically reduced after the synthesis process. Lightly La doped CaB6 single crystals were also synthesized and the La concentration was determined by PIXE.