An apparatus for cleaning a gun barrel having striations formed on an inner surface of the gun barrel includes a cleaning means that is in contact with the inner surface of the gun barrel as being moved by a drive means installed inside a cylindrical main body. The cleaning means includes a main member having a circular plate on which a plurality of insertion holes is formed along its circumference; a rotating member formed inside the circular plate and having a motor and a rotating gear interlocking with the motor; and cleaning members formed to be inserted into the insertion holes, respectively, to clean the inner surface of the gun barrel and having lower sides interlocking with the rotating gear, respectively, to be rotated. The apparatus for cleaning a gun barrel is rotated at high speed with an appropriate pressure along the groove part of the gun barrel accurately, and thus a wide area can be cleaned in a short time. Even if the brusher is worn out, an appropriate pressure is maintained by the spring, and thus the amount of wear is compensated for. Since the brusher does not secede from the barrel, the accuracy is increased, and since the respective brushers are replaceable and it is not required to replace the whole cleaning member, the cost is saved.