A mixed gas supply device includes: a hydrogen gas generation unit that includes a hydrogen generator, the hydrogen generator generating hydrogen gas by decomposition of water and supplying the hydrogen gas; a nitrogen gas generation unit that includes a filter, the filter separating nitrogen gas from air and supplying the nitrogen gas; a gas mixing unit that mixes the supplied hydrogen gas and the supplied nitrogen gas and generates mix gas including the hydrogen gas and the nitrogen gas; and a single base on which the hydrogen gas generation unit, the nitrogen gas generation unit, and the gas mixing unit are mounted, the hydrogen gas generation unit, the nitrogen gas generation unit, and the gas mixing unit being integrated. The gas mixing unit supplies the generated mixed gas to outside.