A flexible tube insertion apparatus includes a flexible tube to be inserted into an insertion target body, a variable stiffness portion that changes a bending stiffness of the flexible tube, a state detection sensor that detects state information of the flexible tube. The apparatus also includes a shape calculator that calculates shape information regarding a shape of the flexible tube based on the state information, an external force calculator that calculates external force information including magnitude of an external force received by the flexible tube based on the state information, and a stiffness controller that controls the variable stiffness portion in accordance with the shape information so that the magnitude of the external force received by the flexible tube becomes smaller than a reference value.