An automatic cleaning apparatus for a gun barrel which includes cleaning means able to be joined to a cylindrical body moving back and forth to the outside and the inside of the gun barrel by transferring rollers connected to and driven by a motor so as to clean the inside of the gun barrel, wherein the cleaning means comprises: a cylindrical tube having a plurality of insertion holes formed along the circumference thereof; a rotary gear connected to and driven by the motor; and cleaning members each of which includes a rotary shaft inserted into the insertion hole and a bevel gear joined to the lower portion of the rotary shaft and interlocking with the rotary gear to rotate the cleaning member, characterized in that the insertion hole includes a small hole having a cross sectional form which is a part of a circle corresponding to the cross section of the rotary shaft, and a large hole communicating laterally with the small hole and having a cross sectional form which is a part of a circle larger than the largest cross section of the lower part of the rotary shaft, and the cleaning means further comprise fixing members each of which includes a downwardly extending portion which has an inwardly dented longitudinal section of a circle having the diameter corresponding to the small hole and which extends in the longitudinal direction of the rotary shaft; and a laterally extending portion which extends from one side of the downwardly extending portion in the lateral direction and is fixed to the cylindrical tube, and wherein each fixing member and each insertion hole are adapted in such a way that the downwardly extending portion can be moved laterally inside the large hole when both the downwardly extending portion and the rotary shaft are inserted into the insertion hole, whereby the fixing member fixes the cleaning member by making the cleaning member come into contact with the small hole when the cleaning member is fixed and does not prevent that the cleaning member is separated to the outside of the cylindrical tube through the large hole when the fixed cleaning member is released from fixation of the fixing member. a cylindrical tube having a plurality of insertion holes formed along the circumference thereof; a rotary gear connected to and driven by the motor; and cleaning members each of which includes a rotary shaft inserted into the insertion hole and a bevel gear joined to the lower portion of the rotary shaft and interlocking with the rotary gear to rotate the cleaning member, characterized in that the insertion hole includes a small hole having a cross sectional form which is a part of a circle corresponding to the cross section of the rotary shaft, and a large hole communicating laterally with the small hole and having a cross sectional form which is a part of a circle larger than the largest cross section of the lower part of the rotary shaft, and the cleaning means further comprise fixing members each of which includes a downwardly extending portion which has an inwardly dented longitudinal section of a circle having the diameter corresponding to the small hole and which extends in the longitudinal direction of the rotary shaft; and a laterally extending portion which extends from one side of the downwardly extending portion in the lateral direction and is fixed to the cylindrical tube, and wherein each fixing member and each insertion hole are adapted in such a way that the downwardly extending portion can be moved laterally inside the large hole when both the downwardly extending portion and the rotary shaft are inserted into the insertion hole, whereby the fixing member fixes the cleaning member by making the cleaning member come into contact with the small hole when the cleaning member is fixed and does not prevent that the cleaning member is separated to the outside of the cylindrical tube through the large hole when the fixed cleaning member is released from fixation of the fixing member.