The present invention makes it easy to control the amount of material gas led out of a tank. Accordingly, carrier gas is introduced into a tank containing a material and together with the carrier gas, from the tank, material gas produced by vaporization of the material is led out. A control part controls the flow rate of the carrier gas so that a concentration index value obtained by measuring mixed gas led out of the tank and indicating the concentration of the material gas in the mixed gas comes close to a predetermined target concentration index value. In addition, the control part controls the flow rate of the carrier gas to change at a predetermined change rate, and then controls the flow rate of the carrier gas on the basis of the deviation between the concentration index value and the target concentration index value.