The insertion device is adapted to hold a high-frequency treatment tool for performing high-frequency current treatment, by means of a distal-end component of a flexible tube for insertion into lumens. The insertion device includes: a first indicator disposed on the front portion of the distal-end component; a first cover formed of an insulating elasto-plastic material and covering the first indicator at the time of the prescribed attachment; an electrical insulation part as a second indicator disposed at the joint portion between the distal-end component and the flexible tube; and a second cover closely contacting the first cover and the electrical insulation part to apply an elastic force to each of them and covering the second indicator at the time of the prescribed attachment so that the leakage of high-frequency currents is prevented.