Recent researches on the beta-alumina solid electrolyte by the present authors have been reviewed. Beta-alumina is a superionic conductor and has a possibility to be used in the many investigations as a solid electrolyte if the chemical stability against the atmosphere or electrode materials have been known. Otherwise, one should consider all the possible reactions between the beta-alumina and atmosphere and electrode materials which are involved to the galvanic cell. Beta-alumina can be applied to detect the SO_x gas, purification of sodium from molten alloys, and thermodynamic investigation of sulfides and iodides as well as molten oxides.