PURPOSE: To make stagnation time and heat treatment uniform and uniformize quality by supplying the powdery materials to be heat-treated such as magnetic metal powder to one end of a spiral path every time when it makes one turn and heat-treating the same while transferring the same along the spiral path.

CONSTITUTION: In heat-treating magnetic metal powder, etc., the apparatus is constituted by a freely rotatable spiral reaction pipe 1, a tunnel furnace 2 for heating, an inlet chamber 20 consisting of a static housing 22 and a rotary housing 23, an outlet chamber 21, a feeder 7, etc., wherein the powdery materials to be treated are supplied by way of the feeder 7, the feed-in pipe 12 every time when the inlet end opening of the reaction pipe 1 faces upward, that is, every time when it makes one turn, and a reducing gas and other reactive gases, and further an inert gas are fed through the gas lead-in pipe 3 of the inlet chamber 20 and are flowed in he reaction pipe. with the above-mentioned apparatus, the powdery materials are uniformly heat-treated and are taken out into a vessel 4. According to this method, the unevening of a fluid state as with rotary kilns does not occur.