A blade driving device includes an opening forming member forming an opening portion, a pivotal member pivoting about the opening portion, blades configured to enter or exit from the opening portion upon receiving power transmitted from the pivotal member, a cover member arranged so that the pivotal member and the plurality of blades are located between the cover and the opening forming member, and engaging portions engaging the of blades with the opening forming member. The pivotal member comes into slidable contact with the engaging portions in a process of pivoting. A pivotal position of the pivotal member with respect to the opening forming member is defined by the engaging portions.