An insertion device includes a thin and elongated insertion section, a rotating body which is rotated to advance or retreat the insertion section, a driving force supply source which supplies a driving force to the rotating body, a variable stiffness section provided for the insertion section and permitting stiffness of the insertion section to be varied, a stiffness detector which detects the stiffness of the insertion section varied by the variable stiffness section, and a controller which controls the driving force supply source in accordance with the stiffness of the insertion section detected by the stiffness detector.