A light amount adjusting device includes: a cover plate having a light-passing hole; a first separator being sheet-shaped and having a hole; a second separator being elastically deformable and sheet-shaped, positioned on a side opposite to the cover plate with the first separator therebetween, and having a hole; a first light amount adjusting blade movable between the cover plate and the first separator; a second light amount adjusting blade being movable between the first separator and the second separator; a base body to which the cover plate, the first separator, and the second separator are mounted, having a throughhole and mounting grooves, the throughhole being formed to have a diameter larger than a diameter of the hole of the second separator, the mounting grooves allowing the second separator to be mounted therein and being opened toward the throughhole.