An apparatus for cleaning a gun barrel having striations, which includes a cleaning device that comes in contact with the inner surface of the gun barrel as it is moved by a drive device installed inside a cylindrical main body. The cleaning device includes (i) a plurality of divided plates that fit atop the striations as a saddle and are slanted to correspond to a slope of the striations, (ii) cleaning members fixed to outer peripheries of the respective divided plates that come in contact with the inner surface of the gun barrel, and (iii) a cam device that converts a rotation of a drive motor installed inside the main body into a rectilinear movement in order to move the respective divided plates upward and downward.