A process liquid changing method is provided for changing process liquids in a substrate processing apparatus including storage tank, a circulation line with a circulation pump, and a process liquid supply nozzle connected to the circulation line through a branch line. The method includes: discharging the process liquid in the storage tank; discharging a process liquid remaining in the circulation line from a drain 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.