PURPOSE: To generate mixed gas having a stable compsn. by introducing respective raw material gases into a heat exchanger and averaging the temps. of the raw material gases supplied to a gas mixture even when temp. difference is generated between the raw material gases.

CONSTITUTION: Liquified argon in an ultralow temp. container 1 is gasified by a fin type heating evaporator 2 or liquified carbon dioxide in of an ultralow temp. container 3 is gasified by an electric heating evaporator 4. The formed argon gas and carbon dioxide are reduced in pressure by pressure reducing valves 5, 6 to be introduced into a mixed gas generator and heat-exchanged by a double-pipe type heat exchanger 7 to be reduced to predetermined gauge pressures by pressure reducing valves 8, 9. After areal type flowmeters 10, 11 are adjusted by main valves 12, 13 so as to respectively become predetermined flow rates, both gases are introduced into a buffer tank 15 through a solenoid valve 14 to produce argon/carbon dioxide mixed gas. Therefore, the variation in the ratio of the supplied gases is suppressed and mixed gas always constant in compsn. is obtained.