Using a new simulative technique developed by us, we systematically investigated new ternary or quaternary molten salt systems, which are based on LiF–LiCl, LiF–LiBr, and LiCl–LiBr binary systems, for use as electrolytes in thermal batteries, and evaluated their ionic conductivities and melting points experimentally. It was confirmed experimentally that LiF–LiBr–KF (melting point: 425°C, ionic conductivity at 500°C: 2.52Scm−1), LiCl–LiBr–KF (405°C, 2.56Scm−1), LiCl–LiBr–NaF–KF (425°C, 3.11Scm−1), LiCl–LiBr–NaCl–KCl (420°C, 2.73Scm−1), and LiCl–LiBr–NaBr–KBr (420°C, 2.76Scm−1) meet our targets for both melting point (350–430°C) and ionic conductivity (2.0Scm−1 and higher at 500°C). A single cell using the newly developed LiCl–LiBr–NaCl–KCl molten salt as an electrolyte was prepared, and the DC-IR of the cell decreased by 20% than that of a single cell using the conventional LiCl–KCl molten salt. It was therefore concluded that the use of new quaternary molten salt systems can improve the discharge rate-capability in practical battery applications because of their high ionic conductivities.