Abstract: The invention provides a gas supply device designed to supply a combustion preventing gas (mixed gas) in which a cover gas for suppressing combustion of molten magnesium held in a melting furnace and a diluting gas for diluting the cover gas are mixed with each other, the gas supply device including a carbon monoxide concentration meter for detecting combustion of a molten metal. The gas supply device further includes a gas introduction device for supplying the combustion preventing gas to the melting furnace in case it is determined that combustion of a molten metal is present, and halts supply of the combustion preventing gas to the melting furnace in case it is determined that combustion of a molten metal is absent.