A device which supplies breathing gas mixed with an additive gas to a patient for pulmonary diagnosis or pulmonary treatment includes a gas mixing unit through which a relatively large flow of breathing gas is achieved from a first gas source through a first pressure regulator and a second pressure regulator. The breathing gas passes through a connecting tube from which the patient can breathe spontaneously through an inspiration tube, thereby inhaling air from the connecting tube. Expired gas passes via an expiration tube to the connecting tube. Additive gas is supplied from a separate gas source via a second pressure regulator and a second flow regulator. The additive gas is supplied in a flow which is considerably smaller than the flow of breathing gas, permitting a very small but accurate concentration of the additive gas to be achieved in the breathing gas. The large flow prevents a reactive additive gas from reacting before being supplied to the patient.