Treatment liquid is sprayed onto a fibrous web surface by one or more spray boxes (21, 11, 23) having a spray chamber (4) and spray nozzles (1) in the travel direction of the fibrous web before a nip (N1 . . . N7; N8; N9) formed between them by two nip means (11 . . . 18) provided with a movable surface, and air and/or treatment medium mist is removed from the spray chamber and after having added treatment liquid the fibrous web is pressed in this nip. The spray chamber (4) is formed between a first spray box (21, 11, 23; 24; 25) and any successive first nip means (13; 52, 53; 62, 63) and second nip means (13; 52, 53; 62, 63), the fibrous web is guided supported by the surface of the first nip means to the spray chamber and the fibrous web is supported thereon during the spraying of treatment liquid.