An ion conducting high-polymer member is formed by an ion containing polymer, and has a circular arc shape. An chord length of the ion conducting high-polymer member is changed. Due to the change in the chord length of the ion conducting high-polymer member, there is change in a relative position of a driving shaft which is coupled with the ion conducting high-polymer member, with respect to a rotating shaft. Due to the change in the relative position of the driving shaft, a circular ring shaped peripheral section is displaced, and due to a rotation of a diaphragm member around the rotating shaft as a center, a part of an aperture is shielded by the circular ring shaped peripheral section, and a diameter of the aperture is changed.