When a gas supplied to a gas injection unit is switched from a first processing gas to a second processing gas, a controller of a gas supply system performs control to open a first supply on/off valve connected to the gas injection unit and provided in a first gas supply line for supplying the first processing gas and a second exhaust on/off valve provided in a first gas exhaust line branched from the first gas supply line, close a second supply on/off valve connected to the gas injection unit and provided in a second gas supply line for supplying the second processing gas and a first exhaust on/off valve provided in a second gas exhaust line branched from the second gas supply line; and then open the second supply on/off valve and the first exhaust on/off valve and close the first supply on/off valve and the second exhaust on/off valve.