A light amount adjusting device includes an opening forming member configured to form an opening portion through which light passes, a blade group configured to move on the opening forming member and form a part of a light passing opening in the opening portion, and a power transmission member arranged outside the opening portion and engaging with the blade group to transmit power. The blade group includes a straight blade configured to move linearly and a swing blade configured to swing from both sides in a straight-ahead operation of the straight blade toward a center of the light passing opening. An edge portion of the light passing opening on a side of the power transmission member or an edge portion on a side opposite to the side of the power transmission member is formed by the blade group.