An insertion device includes an idle state detector detecting an idle state in which an insertion section does not move in directions along the longitudinal axis when a rotation unit rotates in directions around the longitudinal axis, and a movement displacement calculator calculating a rotation direction and amount of the rotation unit based on a driving direction and amount of the driving member and calculating a movement displacement of the insertion section from a reference position in directions parallel to the longitudinal axis based on the rotation direction and amount of the rotation unit. the movement displacement calculator calculates the movement displacement of the insertion section so that a movement amount of the insertion section in the longitudinal directions while the idle state being detected is zero.
