Disclosed herein is an automatic balancing device including a balancer made of a magnetic fluid, a rotatable enclosure having an outer circumferential wall and accommodating the balancer, a magnet provided near at least the center of rotation of the enclosure so as to be rotatable integrally with the enclosure, and a platelike projection projecting from the outer circumferential wall toward the center of rotation of the enclosure, at least a part of the platelike projection extending in a direction different from a radial direction of the enclosure at the juncture between the platelike projection and the outer circumferential wall.