As a part of "the Fundamental Study of Pyrometallurgical Debismuthizing of Lead", the author has studied liquid Pb-Bi, Pb-Na, Bi-Na binaries and Pb-Bi-Na ternary including Pb rich side composition which forms a basis of the Dittmer method. In this study, debismuthizing mechanism of the Dittmer method pertaining to the lead rich corner of the system studied was classified by the results of thermal analysis, solubility determination of Bi and Na in liquid Pb and debismuthizing test by adding metallic sodium. For instance eutectic trough in lead rich corner of Pb-Bi-Na ternary relating to the Dittmer method was determined.