A gas mask is provided for normal-pressure and overpressure operation with at least one breathing port, an expiration valve with a valve spring, an adjusting element affecting the prestress of the valve spring and a pushing element having a first switching position and a second switching position. The pushing element is designed to act on the adjusting element during a translatory motion from the first switching position into the second switching position. A change in the prestress of the valve spring can be brought about and a switchover from a “normal pressure” mode into an “overpressure” mode can thus be carried out.