A mouthpiece for a semi-closed type of breathing apparatus for removing carbon dioxide from breathing air circulating from an inhaling port to an exhaling port in a breathing circuit, comprising a casing, a diaphragm which expands when the inner pressure of a casing is increased through an exhaling operation, exhaling valve member for opening and closing the exhaling port, an urging device for urging the exhaling valve member in such a direction as to close the exhaling port, and a lever having one end linked to the exhaling valve member and the other end linked to the diaphragm, the exhaling valve member being opened through the lever when the inner pressure of the casing is increased through the exhaling operation and the diaphragm expands. The exhaling valve member is urged by the urging device to thereby surely close the exhaling port, so that the exhaling valve is completely prevented from being opened due to the weight of water intruding into the mouthpiece, and thus water in the mouthpiece can be surely prevented from intruding into the breathing circuit.