A treatment instrument insertion device 20 is mounted to the insertion section 2 of an insertion instrument 1 adapted to be inserted into a subject and is used to insert a treatment instrument 31 into the subject. The treatment instrument insertion device 20 has: a tubular treatment instrument guide section 21 for guiding the treatment instrument a guide section body 22 removably provided to the portion of the insertion section, which is located outside the subject when the insertion section of the insertion instrument is inserted in the subject, the guide section body 22 having interconnected therewith one end of the treatment instrument guide section, the guide section body 22 having an insertion opening 22p a diaphragm member 50 which is secured to one end of the guide section body at the portion where the treatment instrument guide section and the guide section body are interconnected, has a hole 50h for the insertion of the treatment instrument, is engaged in a watertight manner with the treatment instrument when the treatment instrument is inserted through the hole, can expand and contract, and consists of a material which prevents the permeation of liquid therethrough and deformation amount restriction sections 22s, 41t for restricting the amount of deformation of the diaphragm member when the diaphragm member deforms in the longitudinal direction of the treatment instrument.Un dispositif dinsertion dinstrument de traitement 20 est monté sur la section dinsertion 2 dun instrument dinsertion 1 conçu pour être inséré dans un sujet et est utilisé pour insérer un instrument de traitement 31 dans le sujet. Le dispositif dinsertion dinstrument de traitement 20 comporte : une section de guidage dinstrument de traitement tubulaire 21 pour guider linstrument de traitement une section de guidage de corps 22 disposée de façon amovible sur la partie de la section dinsertion, qui est située à lextérieur du sujet lorsque la section dinsertion de linstrument dinsertion est
