Abstract: An apparatus for hardening the inside contour of a gun barrel (1) with laser radiation, which apparatus includes a laser, a laser head (10) that can be disposed to be displaced along the bore axis (4) of the gun barrel (1), and beam-deflecting optics (17) disposed in the laser head so that the laser radiation can be coupled into the muzzle of the gun barrel (1) and guided perpendicularly to the inside surface (5) of the gun barrel (1) by the beam-deflecting optics (17). To harden the gun barrel (1) with high precision and homogeneity, and make available a processing space that is relatively large in comparison to known apparatuses, an ND:YAG laser is used as the laser, and, due to the shorter wavelength in comparison to CO.sub.2 lasers, a glass or optical fiber (11) is used for transmitting the laser beam to the laser head. Moreover, the laser head (10) is secured to a drawing rod (3) that can be centered within the gun barrel (1) and displaced in the direction of the bore axis (4) of the gun barrel (1).