PURPOSE: To equally heat a material for effective reaction over the entire length thereof, by providing a kiln for calcining massicot to generate litharge with a heater operating along the entire length of a first tubular structure based on a principle of electromagnetic induction and air supply means for oxidizing the massicot.

CONSTITUTION: A kiln is composed of a single tubular body 2 with a main shaft 3, a heater 4, means 5 for feeding a material to the kiln, screw-conveyer means 6 for translating processed material in the tubular body, means 7 for discharging the material, means 8 for feeding oxidizing air, and means 9 for controlling pressure reduction inside the kiln. The tubular body 2 is composed of a stainless steel pipe equipped with a first head 9 and a second head 10, and the heater 4 comprises an induction coil 11, a thermally-insulated fire- resistant material layer 12 and a ferromagnetic material pipe 13, arranged from the outside toward the inside in this order. The induction coil 11 is composed of a plurality of elements having different numbers of coil turns, so that it can provide various kinds of heating performance.