PROBLEM TO BE SOLVED: To provide an insertion device in which an operation part casing is held without giving a feel of discomfort, and operability of curving operation using the thumb is secured.SOLUTION: In an insertion device, supposing directions perpendicular to a longitudinal axis and a rotational axis of a curving control knob as a first vertical direction and a second vertical direction, a valve operation button is arranged on an external surface facing the first vertical direction in an operation part casing, and a first groove forming surface and a second groove forming surface which form a groove-like part recessed toward the first vertical direction are arranged on an external surface facing the second vertical direction. The second groove forming surface is extended along a rotational track of the curving control knob from a boundary position between tip of the outermost peripheral track of the curving control knob and the rotational axis to a base end direction.
