A thermal treatment furnace includes a thermal treatment chamber in which a thin metal sheet is continuously conveyed horizontally while being floated by air, in which the thermal treatment chamber includes a plurality of air injection nozzles and a plurality of mist spray nozzles that are arranged along a pass line of the thin metal sheet in the thermal treatment chamber, on a lower side and an upper side of the pass line and so as to be orthogonal to the pass line in a side view.