A combination of an endoscope and an endoscope aid configured to be attachably and detachably attached to a treatment tool insertion channel of the endoscope, wherein a suction tube of the endoscope is configured to join the treatment tool insertion channel, the endoscope aid comprising: a flexible tubular member longer than a total length of the treatment tool insertion channel, wherein the tubular member has a treatment tool insertion pipe line that is configured to extend from a proximal end part of the tubular member disposed on an inlet side of the treatment tool insertion channel to a distal end part of the tubular member, and a suction pipe line that is provided separately from the treatment tool insertion pipe line, wherein the distal end part of the tubular member has a sliding contact part that has an outer periphery configured to come in sliding contact with an inner peripheral surface of an outlet portion of the treatment tool insertion channel maintained in the shape of a straight pipe irrespective of bending of an endoscope bending part and is disposed in the outlet portion, and an extending part that is configured to extend from the sliding contact part to a distal end side and is disposed to protrude from an outlet of the treatment tool insertion channel, and wherein the other portion excluding the distal end part of the tubular member is a smaller-diameter part thinner than the distal end part, wherein the suction pipe line is configured to pass through the distal end part in the axial direction such that a distal-end-side opening of the suction pipe line is provided in the distal end surface of the extending part, and a proximal-end-side opening of the suction pipe line is provided in a proximal end surface of the sliding contact part so as to be connected to a gap formed between an inner peripheral surface of the treatment tool insertion channel and an outer peripheral surface of the smaller-diameter part. a flexible tubular member longer than a total length of the treatment tool insertion channel, wherein the tubular member has a treatment tool insertion pipe line that is configured to extend from a proximal end part of the tubular member disposed on an inlet side of the treatment tool insertion channel to a distal end part of the tubular member, and a suction pipe line that is provided separately from the treatment tool insertion pipe line, a treatment tool insertion pipe line that is configured to extend from a proximal end part of the tubular member disposed on an inlet side of the treatment tool insertion channel to a distal end part of the tubular member, and a suction pipe line that is provided separately from the treatment tool insertion pipe line, wherein the distal end part of the tubular member has a sliding contact part that has an outer periphery configured to come in sliding contact with an inner peripheral surface of an outlet portion of the treatment tool insertion channel maintained in the shape of a straight pipe irrespective of bending of an endoscope bending part and is disposed in the outlet portion, and an extending part that is configured to extend from the sliding contact part to a distal end side and is disposed to protrude from an outlet of the treatment tool insertion channel, and wherein the other portion excluding the distal end part of the tubular member is a smaller-diameter part thinner than the distal end part, wherein the suction pipe line is configured to pass through the distal end part in the axial direction such that a distal-end-side opening of the suction pipe line is provided in the distal end surface of the extending part, and a proximal-end-side opening of the suction pipe line is provided in a proximal end surface of the sliding contact part so as to be connected to a gap formed between an inner peripheral surface of the treatment tool insertion channel and an outer peripheral surface of the smaller-diameter part.