Abstract: In the present invention, a gas supply method includes a selecting step and an additive gas supply step. The selecting step involves selecting, in accordance with the type of target film to be processed, a combination of a gas chamber into which additive gas is supplied and the type of additive gas, the gas chamber being selected from a plurality of gas chambers which are divided from a gas injection unit for injecting plasma processing gases into a processing chamber in which a substrate formed with a processing target film is placed. In the additive gas supply step, the additive gas is supplied to the gas chamber on the basis of the combination selected in the selecting step.