To reduce the internal pressure of a vessel while preventing an erroneous detection of a sensor controlling liq. level in a thermostatic vessel by removing foams generating in the thermostatic vessel in recirculating a liq. chemical.

A liq. chemical flowing through a supplying path 10 to a treatment vessel flows while being divided also through a thermostatic vessel return path 11 connected to the midway of the path 10. An antifoaming spray 13 is connected at an end part of the path 11 and a liq. chemical is discharged into a thermostatic vessel 1 from this spray 13. The spray 13 is set in a portion where the liq. chemical flows in from a recovering piping 4. A spray flow rate from the spray 13 is regulated by an opening degree of a flow regulating valve 12 of the antifoaming spray set at the midway of the path 11.