A gas mixture supplying method includes supplying plural kinds of gases through gas supply lines connected to a common pipeline and supplying a gas mixture of the plural kinds of gases from a gas outlet of the common pipeline to a region where the gas mixture is used through a gas mixture supply line. When two or more gases having different flow rates are supplied simultaneously, a gas having a relatively low flow rate is supplied from one of the gas supply lines provided at a position closer to the gas outlet than that for a gas having a relatively high flow rate.