An insertion device includes a UD knob having an outer peripheral side notched in a concave recess with such a curvature and a size as to come in close contact with a base part of a thumb, and an RL knob placed on the UD knob with the same center of rotation, and is configured such that the thumb extends beyond the UD knob, the base part of the thumb is put in contact with a deepest part of the recess of the UD knob, and the RL knob is operably present within a movable range of the thumb beyond the UD knob. The RL knob is configured to be driven by a motor, and thereby the RL knob is rotated by a fingertip to bend a bend portion.
