According to an embodiment of the present disclosure, a process liquid supply apparatus operating method is provided. The method includes filling a filter unit with a process liquid from an upstream side of the filter unit to a downstream side of the filter unit after newly mounting or replacing the filter unit and repeating a depressurization filtering process and a pressurization filtering process for a predetermined number of times. The depressurization filtering process depressurizes the process liquid in the downstream side of the filter unit and thereby allows the process liquid to permeate through the filter unit. The pressurization filtering process pressurizes the process liquid from the upstream side of the filter unit and thereby allows the process liquid to permeate through the filter unit.