A flexible tube insertion apparatus includes an insertion section including a bending portion and a flexible tube portion, variable stiffness sections which causes a change in a level of a bending stiffness of the flexible tube portion, a variable stiffness control section which controls the change in the bending stiffness of the flexible tube portion by the variable stiffness sections, an image sensor which images a subject in a forward direction of the bending portion, a cavity detection section which detects a presence of a cavity in the forward direction based on an image from the image sensor, and a time setting section that sets a time period. The bending stiffness is switched each time period set by the time setting section when the cavity detection section has detected the presence of the cavity in the forward direction.