In a flexible tube insertion apparatus, at least one circuit determines whether an S-shape is formed in an insertion section based on a shape information of the insertion section detected by a detector, determines whether an intersection angle between an extension line of a central axis of the insertion section and a tangential line for the insertion section at an inflection point is enlarged or not based on the shape information, and determines whether or not the stiffness variable portions are provided in a position of the S-shape in the insertion section when it is determined that the S-shape is formed and the intersection angle is enlarged. A stiffness controller increases a stiffness of the stiffness variable portions included in the position of the S-shape in accordance with the determination of the circuit.