An endoscope aid 100 to be attachably and detachably attached to a treatment tool insertion channel 23 of an endoscope 2 includes a tubular member 101 capable of being fixed to an outlet portion 25 in a state where the tubular member 101 is fitted inside the outlet portion 25 of the treatment tool insertion channel 23 that is kept in the shape of a straight pipe regardless of bending of a bending part 15 of the endoscope 2. When the tubular member 101 is fixed to the outlet portion 25, a proximal end 101A of the tubular member 101 located on a side opposite to an outlet side of the treatment tool insertion channel 23 is disposed closer to the outlet side of the treatment tool insertion channel 23 than a proximal end 15A of the bending part 15.