To provide an anticontaminant outer sheath covering method and outer sheath device for an endoscope that can facilitate and smooth the covering of an outer sheath over an endoscope insertion part and the passage of a channel tube in a channel guide, without damage to the outer sheath and the channel tube.

Before an endoscope insertion part 11 is covered with the outer sheath 20, with the outer sheath 20 axially folded into a bellows shape, the channel tube 21 in the outer sheath 20 is passed in an endoscope channel guide 13, and then the outer sheath 20 is extended into a tube shape to cover the endoscope insertion part 11.