A insertion section flexible tube for use in an insertion section of an endoscope includes a flexible tube portion which includes a first core member and a first outer cover which covers the first core member, a bendable portion which includes a second core member and a second outer cover which covers the second core member, the bendable portion having a base end coupled to a tip end of the flexible tube portion; a tightening string for tightening a tip end part of the first outer cover and a base end part of the second outer cover from the outer surfaces thereof; and an adhesive agent for covering and securing the tightening string, wherein at least an outer surface and its vicinity of the first outer cover is formed of a constituent material having low adhesiveness with the adhesive agent, and a region of the outer cover which is covered by the adhesive agent has been subjected to a primary treatment with a primary treatment agent for enhancing the adhesiveness of the outer cover with respect to the adhesive agent.