To provide a gas atmosphere device capable of forming a prescribed gas atmosphere by efficiently discharging unnecessary gas within a container for working a work.

A gas supply port 5 and a gas discharge port 8 are formed in an upper part and a lower part of the container 1, respectively. On the lower side of a stage 10 for installing the work 2, sensors 15, 16 and 17 for detecting concentration of supply gas G1 are arranged. After supply gas G1 of prescribed concentration is filled into the container 1, approaching gas G2 with specific gravity smaller than that of supply gas G1 is stored. When supply gas concentration lower than low concentration determined in advance is detected by medium rank and lower rank sensors 16 and 17, gas supply and gas discharge are performed by a control device 18 through the gas supply port 5 and the gas discharge port 8. When approaching gas G2 is reduced by the gas supply and discharge and the supply gas concentration lower than the low concentration is detected by only the lower rank sensor 17, the inside of the container 1 is maintained to prescribed supply gas concentration by stopping the gas supply and the gas discharge.