An insertion device includes a thin and elongated insertion section (10), a rotating body (14) which is rotated to advance or retreat the insertion section (10), a driving force supply source (18) which supplies a driving force to the rotating body (14), a variable stiffness section (22) provided for the insertion section (10) and permitting stiffness of the insertion section (10) to be varied, a stiffness detector (28) which detects the stiffness of the insertion section (10) varied by the variable stiffness section (22), and a controller which controls the driving force supply source (18) in accordance with the stiffness of the insertion section (10) detected by the stiffness detector (28).
