To provide a blade driving device enabled in the miniaturization of the outer diameter direction and the thinning of the optical axis direction by making an auxiliary member have a plurality of functions.

A blade chamber is constituted of a shutter plate 1 having an opening 1a and a cover plate 2 having an opening 2a so that these openings overlap each other. In the blade chamber, there are arranged: an aperture blade 5 having an opening 5a for regulating a small aperture diameter; shutter blades 7, 8 and 9; and an aperture auxiliary blade 6 that is arranged between the aperture blade 5 and the shutter blades 7, 8 and 9, and has an opening 6a that has an aperture larger than that of the opening 5a and is arranged at a position overlapping the openings 1a and 2a. A blade drive actuator 4 operates the aperture auxiliary blade 6 so that the aperture auxiliary blade 6 covers a gap formed between the aperture blade 5 and edges of the openings 1a and 2a when the opening 5a of the aperture blade 5 is made to enter the opening 6a.