An insertion apparatus comprising: an elongated insertion unit which includes a base (132; 302) in which a central axis is defined, and a rotation member (140; 306) provided on an outer circumference of the base (132; 302) movably in an axial direction of the central axis relative to the base (132; 302) and rotatably around the central axis in response to the rotation of a rotation force transmission portion (202; 342) which is rotatable around a longitudinal axis parallel with the central axis of the base (132; 302); a cylindrical rotation unit which is attachable to the elongated insertion unit and which includes an engagement portion (262; 356) attachable to and detachable from an outer circumferential surface of the base (132; 302); and a press portion (268; 354) which is provided to contact on the rotation member (140; 306) away from the engagement portion (262; 356) and which disposes the rotation member (140; 306) on an outer circumference of the rotation force transmission portion (202; 342) while the engagement portion (262; 356) is engaged with the outer circumferential surface of the base (132; 302), characterized in that the rotation member (140; 306) has a cylindrical member, a diametrically inwardly projecting inward flange at a distal end of the cylindrical member, and that the insertion apparatus further comprises a contact portion formed in the vicinity of the boundary between the cylindrical member and the inward flange so that the press portion of the rotation unit contacts on the contact portion; and the contact portion is inclined diametrically outward and toward a distal side along the central axis; and the press portion (268,354) projects diametrically inward toward the central axis C. an elongated insertion unit which includes a base (132; 302) in which a central axis is defined, and a rotation member (140; 306) provided on an outer circumference of the base (132; 302) movably in an axial direction of the central axis relative to the base (132; 302) and rotatably around the central axis in response to the rotation of a rotation force transmission portion (202; 342) which is rotatable around a longitudinal axis parallel with the central axis of the base (132; 302); a cylindrical rotation unit which is attachable to the elongated insertion unit and which includes an engagement portion (262; 356) attachable to and detachable from an outer circumferential surface of the base (132; 302); and a press portion (268; 354) which is provided to contact on the rotation member (140; 306) away from the engagement portion (262; 356) and which disposes the rotation member (140; 306) on an outer circumference of the rotation force transmission portion (202; 342) while the engagement portion (262; 356) is engaged with the outer circumferential surface of the base (132; 302), characterized in that the rotation member (140; 306) has a cylindrical member, a diametrically inwardly projecting inward flange at a distal end of the cylindrical member, and that the insertion apparatus further comprises a contact portion formed in the vicinity of the boundary between the cylindrical member and the inward flange so that the press portion of the rotation unit contacts on the contact portion; and the contact portion is inclined diametrically outward and toward a distal side along the central axis; and the press portion (268,354) projects diametrically inward toward the central axis C.