PURPOSE: To perform a heat treatment using a high-cleanliness reaction gas in a heat-treatment region by filling the surroundings of a reaction tube always with a gas after reaction with almost the same composition as that of the reaction gas atmosphere in the reaction tube to prevent impurity components entering the reaction tube by permeating the wall of the reaction tube from existing.

CONSTITUTION: A reaction gas is fed to a reaction gas introduction tube 8 from a reaction gas supply source. The reaction gas is released to the heat- treatment region 3 in the reaction tube 5 from respective injection ports 8 of the reaction gas introduction tube 8. After reacting on a treated substrate 9, the released reaction gas goes from a reaction gas-passing hole 4 into a reaction gas introduction region 6 formed by an external tube 7 to surround the outer periphery of the reaction tube 5. As a reslt, the external atmosphere of the reaction tube 5 is filled with a reaction gas atmosphere after the end of a reaction treatment, the compositions of the reaction gases in and outside the reaction tube 5 are almost the same and no impurity permeating the tube wall of the reaction tube exists so that the inside of the reaction tube is always occupied only by clean reaction gas components.