Exhalation valve including a diaphragm for opening and closing an outlet of an exhalation flow path for guiding exhaled air to the outside air, a back chamber provided opposite the exhalation flow path in the diaphragm and forming a space together with the diaphragm, and a pump unit fixed to the circumference of the back chamber for adjusting the air pressure inside the back chamber by feeding and discharging of air to and from the back chamber. The diaphragm closes the outlet of the exhalation flow path when the air pressure inside the exhalation flow path is lower than the air pressure inside the back chamber, and opens the outlet of the exhalation flow path when the air pressure inside the exhalation flow path is higher than the air pressure inside the back chamber.
