A balancer, attachable via its attaching portion to a rotatable body, and rotatable together therewith about its rotation axis, includes: weights of same mass and shape; and a weight holder with storage chambers for housing the weights, respectively, such that the weights can oscillate in arbitrary directions. The weight is shaped similar to and smaller than the chamber. The chamber is a cylindrical space extending alongside its central axis; and has concave curved ends perpendicular to the central axis. Midpoints of the central axes of the chambers are contact points on a circumference of a circle having the rotation axis as its center and being on an arbitrary plane perpendicular to the rotation axis; and are positioned so that they create a rotational symmetry around the rotation axis. The balancer can correct dynamic imbalance in rotatable bodies, such as machine tools, at both low-speeds and high-speeds of rotation.