PROBLEM TO BE SOLVED: To control an expiratory valve accurately.SOLUTION: An expiratory valve 15 includes: a diaphragm 22 which opens/closes an outlet of an expiratory flow passage 13 guiding the expiration to the atmosphere a back chamber 23 which is formed opposite to the expiration flow passage 13 in the diaphragm 22 and forms a space with the diaphragm 22 and a pump unit 25 which is fixed to a periphery of the back chamber 23, supplies and exhausts air in the back chamber 23 and adjusts an air pressure in the back chamber 23.
