To easily heat treat granular substances with a device of simple construction by inserting a tray on which a vessel in which the granular substances have been thrown is rotatably supported into a furnace and heat treating the granular substances while the vessel is rotated by a driving member in the furnace.

After granular substances are thrown in a vessel 71 rotatably supported on a tray 51 through rollers 61a, 61b from a feeding port 81a, the tray 51 is inserted into a heating chamber 11b of a heating furnace. Next, a driving member 41a and an alignment member 41b are moved forward to fit key pieces 91a, 91b in keyholes of the driving member 41a, and also the alignment member 41b is fitted in a recessed part of a one side face 71b, and the vessel 71 is rotated by a driving motor through the driving member 41a. At this time, by heating the inside of the heating chamber 11b by heaters 21a, 21b in advance to keep it heated, the granular substances in the vessel 71 are heat treated while rolled. And when a vacuum pump is connected to a furnace shell 11 through a condenser, the granular substances can be heat treated under the reduced pressure.