A method and apparatus, for supplying high-pressure mixed gas of a low-vapor-pressure first gas as an active gas and a high-vapor-pressure second gas, are arranged to reduce an amount of the first gas discarded. The mixed gas in a high-pressure state is supplied from a mixing container to a use point. Upon reduction of pressure in the mixing container to a setpoint as a result of supply to the use point, a predetermined amount of the first gas is charged into a replenishment container connected to the mixing container by a replenishment line having a replenishment valve, and which is evacuated. As the second gas is charged into the replenishment container charged with the first gas, the replenishment valve is opened such that the first gas in the replenishment container is forced out by the second gas, thereby charging the mixing container with the mixed gas in the high-pressure condition.