PURPOSE: To improve the focusing performance of a video camera, etc., equipped with an automatic focus detecting device by forming stop blades and a light quantity attenuating means in such a shape that even when the aperture ratio varies, the aperture part consisting of the both is invariably single.

CONSTITUTION: The small-aperture side shapes of the stop blades 1 and 2 each consist of two straight line parts so that the shapes of the aperture part 4 at the time of intermediate aperture and small aperture are rectangular and symmetrical about a straight line 10 which crosses the optical axis and is perpendicular to the sliding directions of the stop blades 1 and 2. Namely, the stop blades 1 and 2 and light quantity attenuating means 6 are so shaped that even when the aperture ratio varies, the aperture part 4 consisting of the stop blades 1 and 2 and light quantity attenuating means 6 is invariably single. Consequently, there are not plural aperture parts 4 and the focusing performance of the video camera, etc., equipped with the automatic focus detecting device which performs focusing according to the output voltage of an image pickup element can be improved.