A raw material gas supply method for use in a film forming apparatus which forms a film on a substrate, includes supplying a carrier gas to a gas phase zone defined inside a raw material container accommodating a liquid or solid raw material, vaporizing the raw material, supplying a raw material gas containing the vaporized raw material from the raw material container to the film forming apparatus via a raw material gas supply path, measuring a flow rate of the vaporized raw material flowing through the raw material gas supply path, comparing the flow rate of the vaporized raw material obtained by the flow rate measurement unit with a predetermined target value, and controlling an internal pressure of the raw material container to be increased when the flow rate is higher than the predetermined target value, and to be decreased when the is lower than the predetermined target value.