A cleaning brush which is inserted into a conduit of the endoscope, includes a long and flexible shaft, and a brush part provided at a leading end of the shaft. The brush part includes brush bristles, a strand, etc. A part of the shaft that overlaps the brush bristles in a state where the brush bristles are bent toward the leading end of the shaft, is provided with a small diameter part that is smaller in external diameter than the other part of the shaft. The dimension obtained by adding the external diameter of the small diameter part and twice the external diameter of the brush bristles is smaller than the internal diameter of a conduit into which this cleaning brush is inserted.