Abstract: A damage evaluation apparatus, which is used on a concrete structure having an embedded tendon to be evaluated for damage. The damage apparatus includes a magnetizer for generating magnetic force, and a detector for detecting a change in magnetism produced from a damaged area of the tendon. The magnetizer includes an excitation coil; an iron core passed through a center hole of the excitation coil; and a pair of columnar yokes connected to respective ends of the iron core and each extending toward the surface of the concrete. By passing an electric current through the excitation coil, a magnetic circuit is formed by the yoke shaft, the pair of columnar yokes, and the tendon over a range thereof situated between a pair of plate-shaped yokes. Current that flows through the excitation coil is controlled such that the magnetic flux density of the tendon is rendered constant.