Abstract: In an endoscope system, a nasal endoscope for entry in a first external nostril of a patient includes a first elongated tube, a first distal portion thereof, an imaging window, an instrument channel, a handle section, and a steering section. An assist device for entry in a second nostril includes a second elongated tube, a second distal portion thereof, and a working channel with a greater inner diameter than the instrument channel. A retaining device retains the second distal portion on the first distal portion removably and equidirectionally, to cause a second distal end surface to contact a lower peripheral part outside a first distal end surface lower than a horizontal line passing a center point of the imaging window when the first distal end surface is viewed in a state of orienting an upper side of steering of the steering section in an upward direction.