A gas supply system for supplying a gas into a processing chamber for processing a substrate to be processed includes: a processing gas supply unit; a processing gas supply line; a first and a second processing gas branch line; a branch flow control unit; an additional gas supply unit; an additional gas supply line; a first and a second additional gas branch line; a flow path switching unit; and a control unit. Before processing the substrate to be processed, the control unit performs a pressure ratio control on the branch flow control unit while the processing gas supply unit supplies the processing gas. After the inner pressures of the first and the second processing gas branch line become stable, the control unit switches the pressure ratio control to a fixed pressure control, and then the additional gas supply unit supplies the additional gas.