A flexible tube insertion device includes a flexible tube section, at least one variable stiffness unit, at least one state detector, and a stiffness controller. The flexible tube section is segmented along an axial direction into segments and configured to be inserted into an insertion target. The variable stiffness unit is configured to vary bending stiffness of the flexible tube section in units at least one segment. The state detector is configured to detect information relating to shape information of the flexible tube section. The stiffness controller is configured to cause the variable stiffness unit to reduce bending stiffness of a portion of the flexible tube section including a place where the flexible tube section is easy to bend in units of at least one segment based on the shape information.
