A process liquid supply system includes a pure water supply source connected to a process vessel through a supply pipe; a chemical liquid tank that stores a chemical liquid and supplies the chemical liquid stored therein to the supply pipe; and a compressed gas supply source connected to the chemical liquid tank through a compressed gas supply pipe. A pressure adjusting part is disposed on the compressed gas supply pipe at a position between the compressed gas supply source and the chemical liquid tank. The pressure adjusting part is controlled so that the pressure of the compressed gas supplied from the compressed gas supply source to the chemical liquid tank is gradually or stepwise increased after the supply of the chemical liquid from the chemical liquid tank to the supply pipe is started so that the concentration of the processing liquid remains at desired level.