Abstract: An improved apparatus for producing potassium sulfate and hydrogen chloride gas from potassium hydrogen sulfate and potassium chloride is provided, which comprises a horizontal type muffle furnace having an arcuate upper wall, at least one shaft of an agitator, a means for feeding raw materials, and a means for discharging the reaction product, to discharge the product by overflow; and an outer casing for said muffle furnace in which said furnace is accomodated on a bed of an insulating material and a space over said furnace, as a passage for heating gas. A heating gas introduced into said space over said furnace supplies the heat quantity required for the total reaction, through the upper wall of the furnace to raw materials. According to this apparatus, the life of furnace for continuous operation can be prolonged.