Part of an atmosphere gas in a heating zone 1 and/or a soaking zone is sucked out and is cooled through a high-temperature gas passage of a heat exchanger 9 by heat exchange with a gas in a low-temperature gas passage, is then cooled through a gas cooler 10, is then dehumidified to a dew point of -45°C or less in a dryer 11, is then heated through the low-temperature gas passage of the heat exchanger 9 by heat exchange with a gas in the high-temperature gas passage, and is returned to the heating zone 1 and/or the soaking zone. Part of gas flowing from the dryer 11 toward the low-temperature gas passage of the heat exchanger 9 is returned to a cooling zone 2. These can achieve a low dew point of -45°C or less with high energy efficiency.
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