Authors have been trying to fabricate cristobalite-containing mullite bodies with high strength and toughness by reheating mullite-glass sintered bodies. In this process, a phenomenon that once-crystallized cristobalite layer changed back to amorphous again was observed by the microstructure observation of the sample contaminated with Na₂O. In this study, the minimum amount of Na₂O to change the cristobalite phase into amorphous was quantitatively determined by the electron probe microanalysis (EPMA) method. As a result, the amount of Na₂O in the amorphous phase just near the cristobalite-amorphous phase boundary was 1.6-1.9 mass%, which almost corresponded with the minimum amount of Na₂O in the liquid phase coexisting with mullite, estimated from a phase diagram.