An apparatus for treating the surface of an object to be treated comprising introducing a surface treatment fluid into a reaction vessel (4) capable of receiving an object, introducing the surface treatment fluid into a separation vessel (14) after the object is subjected to surface treatment, and circulating the surface treatment fluid, from which a contaminant has already been removed, to the reaction vessel (4). At the time of treatment on the surface of the object, a circulation passage for the surface treatment fluid including the reaction vessel (4) is communicated and the surface treatment fluid is constantly circulated through the circulation passage.