A gas supplying unit supplies a nitrogen gas into a furnace body of a graphite heating furnace in which at least a part of the furnace body is formed with a graphite. An exhausting unit exhausts a gas inside the furnace body to outside the furnace body. A dew-point temperature of the nitrogen gas supplied into the furnace body is equal to or lower than −80° C. A pressure inside the furnace body is equal to or higher than 140 Pa with respect to an atmospheric pressure outside the furnace body.