A source gas supply apparatus that supplies a source gas into a processing container, includes: a raw material container configured to contain a raw material, and to vaporize the raw material; a source gas supply flow path configured to supply the source gas including the vaporized raw material into the processing container; a flow rate measurement part installed in the source gas supply flow path, and configured to measure a flow rate of the source gas; a diluent gas supply flow path joining a downstream side of the flow rate measurement part in the source gas supply flow path, and configured to supply a diluent gas for diluting the source gas; and a gas mixer provided at a merging portion of the source gas supply flow path and the diluent gas supply flow path, and configured to mix the source gas with the diluent gas via a Venturi effect.