An insertion device includes a support member supporting a rotating tubular member between a base member and the rotating tubular member in diametrical directions and holding the rotating tubular member at a position where a rotation driving force is transmittable from a driving unit to an inner peripheral portion of the rotating tubular member connected to the driving unit. The insertion device includes a distal side ring member maintaining liquid-tightness between an outer peripheral portion of the base member and the inner peripheral portion of the rotating tubular member to a distal direction side of the driving unit placement cavity, and a proximal side ring member maintaining liquid-tightness between the outer peripheral portion of the base member and the inner peripheral portion of the rotating tubular member to a proximal direction side of the driving unit placement cavity.