A gas supply unit, for supplying a gas into a processing chamber in which a substrate is processed, includes a plurality of gas supply sources, a mixing line for mixing a plurality of gases supplied from the gas supply sources to make a gaseous mixture, a multiplicity of branch lines for branching the gaseous mixture to be supplied to a multiplicity of places in the processing chamber, and an additional gas supply unit for supplying a specified additional gas to a gaseous mixture flowing in at least one branch line. The gas supply unit also includes pressure gauges and valves for adjusting gas flow rates in the branch lines, respectively, and a pressure ratio controller for controlling that gaseous mixtures branched into the branch lines to have a specified pressure ratio by adjusting opening degrees of the valves based on measurement results obtained by using the pressure gauges.