PURPOSE: To manufacture highly reliable mixed gas at low cost by holding the respective gas concn. values of the mixed gas at certain specific values with high precision, and speedily returning each gas concn. value to the specific value and stabilizing it even if the concn. varies.

CONSTITUTION: This atmosphere control furnace consists of plural flow rate control means 1 which controls the flow rates of respective gases, a gas mixing furnace 2 wherein the respective gas sent from those flow rate control means 1 are mixed, gas sampling means 3a and 3b which are provided at the entrance 2a and exit 2b of the gas mixing furnace 2, gas sensors 4a and 4b which are connected to the gas sampling means 3a and 3b and measure the respective gas concn. values, and a control means 5 which controls the respective gas concn. values.