To solve a problem wherein a blurred image of a background becomes rhombus shape if a shape of a light passing port in a small aperture state is made a rhombus.
In the diaphragm mechanism 10, two aperture blades 14 and 16 are arranged to be superimposed on each other, and one light passing port 50 for introducing light to be used for imaging is formed by both edges of the aperture blades. A driving mechanism 22 drives the two diaphragm blades 14 and 16 in different directions from each other, whereby opening area of the light passing port 50 is changed within a prescribed range. The two aperture blades 14 and 16 are provided such that the one light passing port 50 surrounded by a boundary including eight lines is formed by the edges of both of them when they are respectively located at the minimum aperture position where the opening area of the light passing port 50 becomes the minimum in the prescribed range.
ONO TAKASHI