A respiratory valve, comprising a valve housing with a chamber, inlet (21) and respiration (22) openings. The chamber is limited by an arcuated or aslant interior wall section (18) extending from a lower part and into an upper part of the chamber, and is terminated in the upper part of the chamber, under the respiration opening. An outlet opening (23) is provided in the interior wall section. An elongated valve flap (32) in the form of a resilient plate element fixed to the lower part of the chamber terminates in the upper part of chamber. The flap is wide enough to cover the outlet opening, and is adapted to be positioned, in a rest position, in a distance from the interior wall section, and, in a work position, in contact with a part of the interior wall section. In the work position, the flap substantially covers and closes the outlet opening, and establishes a passageway between the inlet and respiration openings.