A processing apparatus is provided with a showerhead for introducing a process gas into a processing vessel, and heaters for heating the showerhead to an elevated temperature. The processing apparatus is provided with a main flow line for a cooling liquid, and a bypass flow line connected to the main flow line so as to bypass the showerhead. A cooling liquid control system for controlling the flow of the cooling liquid includes shutoff valves, and a valve controller. The cooling liquid control system controls the flow of the cooling liquid such that the cooling liquid is prevented from flowing through the cooling passage while the showerhead is being heated to the elevated temperature, and such that the cooling liquid is made to flow through both the cooling passage and the bypass flow line while the showerhead is being cooled to a reduced temperature.