An insertion device sets engage range which it prescribed in return power to the neutral position by elastic force of the spring and rotation resistance force by the slide power by elastic member for dial portion performing curving operation of a curving portion. If a rotary angle is beyond an engage range, the dial part lets a rotary point of view to order return in an engage range and, if there is a rotary angle in an engage range, the dial part keeps a target part in an observation field of vision.