A light quantity adjusting device includes a ring-shaped base plate having an exposure aperture at a center thereof, a first light quantity adjusting blade arranged in the exposure aperture, a first drive device for opening/closing the first light quantity adjusting blade, a second light quantity adjusting blade arranged in the exposure aperture, and a second drive device for opening/closing the second light quantity adjusting blade. The first drive device includes a magnet rotor and an exciting coil formed in a ring shape, and a guiding member for inducing a magnetic field to a position facing a magnetic pole formed on a circumference side of the magnet rotor. The second drive device includes a magnet rotor, an exciting coil, and a guiding member for guiding a magnetic field to a position facing a magnetic pole formed on a circumference side of the magnet rotor.