Process liquid changing method and substrate processing apparatus

A process liquid changing method for changing process liquids in a substrate processing apparatus, said apparatus including a storage tank configured to store a process liquid, a circulation line having ends thereof connected to the storage tank, a circulation pump provided on the circulation line, and a process liquid supply nozzle connected to the circulation line through a branch line, said process liquid changing method comprising:

• discharging the process liquid in the storage tank;
• discharging a process liquid remaining in the circulation line from a drain part connected to the circulation line at a second position of the circulation line, while supplying a purge gas to the circulation line at a first position of the circulation line, wherein the first position is located upstream of a junction area where the branch line is connected to the circulation line, and the second position is located downstream of the junction area and upstream of the storage tank:
• and
• supplying a process liquid into the storage tank.