Provided is an endoscope auxiliary tool whereby the inside diameter of a treatment tool insertion channel of an endoscope can be changed in accordance with the situation while suppressing a reduction in bending performance of an endoscope bending part, and disposition of the treatment tool in the exit portion of the treatment tool insertion channel can be stabilized, and also provided is an endoscope provided with a treatment tool insertion channel to which the endoscope auxiliary tool can be attached. An endoscope auxiliary tool 100 detachably attached to a treatment tool insertion channel 23 of an endoscope 2 is provided with a tubular member 101 which can be fixed in an exit portion 25 of the treatment tool insertion channel 23 in a state of being fitted inside the exit portion 25, the exit portion 25 being maintained in a straight-tube shape regardless of bending of a bending part 15 of the endoscope 2, and when the tubular member 101 is fixed in the exit portion 25, the proximal end 101A of the tubular member 101 positioned on the opposite side from the exit side of the treatment tool insertion channel 23 is disposed further toward the exit side of the treatment tool insertion channel 23 than the proximal end 15A of the bending part 15.L'invention concerne un outil auxiliaire d'endoscope, le diamètre intérieur d'un canal d'insertion d'outil de traitement d'un endoscope pouvant être changé en fonction de la situation tout en supprimant une réduction de la performance de courbure d'une partie de courbure d'endoscope, et la disposition de l'outil de traitement dans la partie de sortie du canal d'insertion d'outil de traitement pouvant être stabilisée, et l'invention concerne également un endoscope pourvu d'un canal d'insertion d'outil de traitement auquel l'outil auxiliaire d'endoscope peut être fixé. Un outil auxiliaire d'endoscope 100 fixé de manière amovible à un canal d'insertion d'outil de traitement 23 d'un endoscope 2 est pourvu d'un élément tubulaire 101 q
