A gas supply apparatus supplies a gas to a processing space where a gas processing is performed on a substrate. The gas supply apparatus includes: a gas supply source configured to supply a gas; a gas supply path configured to supply the gas to the processing space; an opening/closing valve configured to supply/stop the gas and provided in the gas supply path; a detector configured to detect a detectable index correlated with a Cv value of the opening/closing valve; an opening degree adjustment mechanism configured to adjust an opening degree of the opening/closing valve when the opening/closing valve is opened; and a controller configured to: store a relationship between the Cv value and the index; and control the opening degree by the opening degree adjustment mechanism such that when the index deviates from an appropriate range corresponding to an appropriate Cv value, the index falls within the appropriate range.